Universe: AU. An AU, somewhere between the DCAU and the comics. Kara Zor-El and Cassandra Cain are both from the comics, but from a place before Infinite Crisis and OYL. General continuity-wise, things are closer to the cartoons than the comics, though, and this isn't set in any specific comic arc (in fact, to try would drive me absolutely bonkers). The Joker is straight out of TAS.
Spoilers: …Caveat lector.
Disclaimer: Batgirl, Supergirl, and every other character or thing in this story are the property of DC Comics, Warner Brothers or both. No copyright infringement is intended or implied.
Author's Notes: I've been wanting to write this story for such a long time, but I never really had the motivation to actually finish it. Except that now, I do. Go me.
Also, Gotham may be in Anystate, USA in the comics, but for my story, I am superimposing it over real life Delaware City. I need a fixed location in my head in order to write and New York has been done to death. I don't think this actually comes into the story at all, but there you go.

Kara Zor-El pulled her punch perfectly this time, barely connecting with Livewire's face. She only flew about three feet through the air and two inches off the ground before landing in an undignified heap at Kara's feet.

Livewire tried to raise her head, but failed miserably. Kara hauled her up by her collar and dropped her off with the Metropolis MCU before flying away.

She made her course for Smallville, Kansas, taking her time. She found flying to be one of the most enjoyable parts of her life on earth, so she spent a good forty-five minutes on a leisurely flight.

But she did, at last, enter a farmhouse that held three individuals. Two were entirely Human, and one was a cloned half-Kryptonian. Kara climbed through her window and was out of her costume and into some jeans and a tee-shirt in about two seconds.

She found Kon-El in the office with Uncle Jonathan (as she'd finally settled on calling him, given that he was her cousin's father). Jonathan Kent was reading an article on soil management, irrigation, and fertilization, but he looked up when she entered.

"Hey, Kara," Uncle Jonathan said. "How was Metropolis?"

"It was… I dislike it," she said.

"Oh?" Kon-El asked.

"It is very, very loud. I can not turn in any direction without hearing unending noise," Kara sighed.

Uncle Jonathan smiled. "Clark occasionally mentions he hears a bit too much. I've always gotten the impression he was understating the case by a great deal."

Kara glanced at Kon-El who seemed a bit amused but said, "So, you have finished work for today?"

"Sorry," Uncle Jonathan said with a grin. "Although, we're trying to decide where to rotate everything for next year. Any thoughts?"

Kara tried to think of the most appropriate response when he said, "I'm sorry, Kara. I…it seems so natural having you here that sometimes I forget you didn't really grow up on Earth."

Kara sighed. "It must seem so stupid. I'm grieving for a planet that's been dead for over thirty years."

"Don't think that," Uncle Jonathan said.

"No way!" Kon said at the same time. He frowned and then said, "Kara, there aren't really a lot of people who can identify with you. J'onn is the only one who comes to mind. Even Clark…he was too young when he left Krypton to really remember it. But you… You don't need to feel like you owe it to us to jump in with everything. I may not get it, but I kind of get it."

Kara gave him a small smile.

Uncle Jonathan nodded. "You're welcome to take all the time you need. Kara, you've been through a lot. I'm surprised you're even doing the whole…what do we call it? The super-hero thing?"

"It seemed like such a good idea," she sighed. "I like helping people, but…I can't do enough. And it's so hard not to hurt anyone. I have so much power. What if I create more damage than I stop?"

Uncle Jonathan pulled a copy of the Intrigue section of the Daily Planet out of a drawer. It was folded so that the back page was showing. The "Super-watch" section. Near the bottom was the article he obviously wanted her to see.

METROPOLIS – MEET SUPERGIRL

METROPOLIS – A new figure wearing a red S has been seen in the Metropolitan skies lately. Supergirl, as she's being called, may not have quite the following of the Man of Steel yet, but she's already making a name for herself.

"I can remember when Superman first started out here in Metropolis. It's so exciting to see someone else starting out," said Miguel Travieso, a detective in the Metropolitan Police Department.

Others were less enthusiastic. "I am just not comfortable with so many meta-humans in the city. Superman and Superboy can see through walls and hear noises from across the city. By all indications, so can this newcomer. How do we know we can trust her?" said James Taggart, an employee of Wells Fargo.

So far indications are that Supergirl intends to follow in Superman's footsteps. Already she's been sighted performing daring rescues across the city, and has assisted in the apprehension of at least twenty suspected criminals.

Souveniers of the new figure are cropping up all around the city. Anna Richardson, aged thirteen, was interviewed buying a poster of Supergirl and said, "I would be dead right now if it weren't for her. I almost drowned and she saved my life. I hope she swings by my house sometime. I really want to say thank you."

Next to the article was a picture of her. She had apparently just taken off over one of the city's parks.

"I do not…look like this.," Kara said.

Uncle Jonathan gave her a rueful smile. "You may not do perfectly forever. But so far, you're doing alright."

Kara's phone went off. She looked at it and then swore in Kryptonian.

"Language, young lady," Uncle Jonathan said.

"I am sorry. But Luminus has escaped," she reported.

"Already?" asked Kon-El. "He was only locked up again last week. How?"

"One of the officers that was handling him is apparently new to Metropolis. And he is believed to be headed…oh."

"What?" Uncle Jonathan asked.

"Gotham City. He is going to Batman's city," Kara said. If there was anyone she never wanted to see again, it was Batman. His stern disapproval practically echoed, it had been so loud. He had a way of looking at her that made her feel like she owed him something, even when she'd done nothing wrong.

She hated him.

"No sweat," Kon-El shrugged. "I'll call Robin."

His phone was in his hand and dialed in a split second. When Robin picked up, Kara could clearly hear both sides of the conversation.

"Hello?"

"Robin. Are you okay for shop talk?" Kon-El said.

"Yeah, Conner! It's good to hear from you, but I'm in between some stuff. I'm in Ireland!" was Robin's voice, too high and too excited. He was probably with someone who wasn't in the loop. Or in public, judging by the background noise. "What's up, though?

"Da…rn." Kon-El ended the word differently at a stern look from Uncle Jonathan. Kara grinned. Kon-El continued, " I am here with Supergirl, who tells me that Luminus is headed into Gotham city as we speak. Are you…I hate asking questions you can answer in code. Are you in Ireland with Batman?"

"We're both here until the end of the week."

"Nightwing?"

"No, he didn't come with us. I think he's visiting some friends, though. He's definitely not at home."

Kon-El pulled back a moment to glare irritably at his cell phone before saying, "Okay, what about Batgirl? Or that Oracle thingy?"

"You should definitely call them. They were so upset they didn't get to come and I think they're feeling a little lonely, but we didn't think it would be responsible for everyone to leave," Robin said. Then he said to someone else, "Oh, hey! Conner's on the phone."

A new voice answered Robin on the other end. "Oh? I hope everything's alright. Tell him I said hello."

"Batman's with you now," Kon-El guessed.

"Yeah, that's him. He says 'Hi,'" Robin said.

"Batman says hi. A sentence never before uttered in the English language. Okay, so Batgirl and Oracle are both in Gotham right now?"

"Yeah."

"But no one else? What about Huntress or Black Canary?"

"They're both on a business trip overseas," Robin replied.

"A what?" Kara asked.

"Probably Justice League business," Kon-El shrugged.

"I keep forgetting you guys hear the other side of phone conversations," Uncle Jonathan said, shaking his head.

"Okay. We'll call Superman and see what he has to say about things," Kon-El finally said.

"Sounds good. I can't wait to talk to you when I get back. Want anything from over here?" Robin asked.

"What? I don't know! Surprise me."

Kon-El and Robin said their goodbyes and hung up. He called Kal-El next.

"Conner. Good to hear from you. Lois and I were just wondering when you were coming up to visit," came Kal-El's voice.

"Hey, Clark," Kon-El said.

"Hey, Clark," Uncle Jonathan echoed.

"Hello, Kal-El," Kara said.

Kal-El laughed on the other end of the phone. "Hey to Pa and Kara."

"Listen, we've got a work-related problem. Are you free to talk?" Kon-El asked.

"Yeah, I'm at home," Kal-El replied.

"Okay, Luminus is headed to Gotham and the only people there right now are Batgirl and Oracle. Apparently he caught some kind of lucky break with the police because he was gone almost as soon as he was in custody," Kon-El explained.

"I see. Unfortunately, I've got my hands full right now in Metropolis and the League. And you're on the rotation with the League this month from the Titans, correct?" Kal-El said.

"That just about covers it," Kon-El agreed.

"Well, then, Kara. It looks like you're going to Gotham City," Kal-El said.

Kara sighed. Out loud she said, "I'll leave right away. Should I announce my presence to anyone when I arrive?"

"Find Batgirl. She's the one you'll be backing up, and Gotham tends to work a bit differently than you'd think it ought to," Kal-El told her.

"Alright."

"Thanks, Clark," Kon-El said. He finished the call and closed his phone.

"I guess I should go change, then," Kara sighed.

"Kara," Kon-El said. "The Bat-clan…they come across all grim and moody, but they're actually okay. Don't let Batgirl scare you off with the attitude."

"Why would I be frightened of her? Can she harm me?" Kara asked curiously.

"Probably, but she won't unless you're exposed to red kryptonite or something," Kon-El said. Kara tried not to be unnerved by the certainty in that "probably." She was not accustomed to humans that could damage her.

"Look," Kon-El continued, "it's just…the Bats are weird. And they're weird about their city. Don't let her get to you."

"I will…bear that in mind." Kara walked up the stairs at human speed, but seconds later was back in her costume and soaring out of the window and then over the country below. The golden fields of the Midwest deepened into the lush green of the Mississippi River Basin before the peaks of the Appalachians rose towards her. And on the other side of that, the Eastern Coast of the United States and Gotham City.

Cassandra Cain was hunched over against the rain, watching Harley Quinn slink through Gotham's back alleys. She had those two cackling hyenas with her and Cassandra had picked up her trail as she escaped from her most recent crime—robbing a drug store.

The crime waves that plagued Gotham City whenever one of the major criminals escaped from Arkham were all totally unique. Which meant they had unique requirements in weapons and equipment. Batman had put together a system for spotting who was gearing up and what for a long time ago. The Joker, famed for his wide array of toxins and gasses, tended to stock up on pharmaceuticals before implementing whatever terrorist plot he was planning next. Cassandra had been planning to start tracking down this "Luminus" character that Batman and Robin had said was headed into Gotham when Harley had nearly run straight into her, escaping from that drug store. Batman had once told her, "Luck is the most unreliable tool in a detective's arsenal, but it can be the most effective one. If you happen onto a lead, don't give it up unless you're absolutely forced to."

Harley was humming to herself as she made her way into the Cheshire Arms Hotel. There was an enormous grinning cat on the sign. Cassandra didn't understand the reference. She'd ask Alfred later maybe, although if it were from a book that might set him off again. Alfred was constantly after her to read more books. Right now he had her struggling through The Merry Adventures of Robin Hood by Howard Pyle. And she hated to admit it, but it actually was enjoyable.

Cassandra snuck inside, to watch Harley and the Joker meet up. It was more than her costume was worth to try taking on the Joker alone, but she had every intention being able to tell Batman exactly where the Joker was and what he was up to just as soon as Batman got back.

Ireland had better be worth it.

They were speaking but Cassandra ignored this. Their body language was much more useful. She settled herself on an open hallway that overlooked an interior lobby, so as to get a good view.

Harley's legs were tired, her hands were the same, but the rest of her was desperate that the Joker would approve of her. She gave him the duffel with her take in it, with arms that begged for him to touch her, to smile at her.

The Joker actually was quite happy with Harley. His whole body had taken on a pleased attitude when she entered the room. Cassandra had the suspicion that Harley Quinn was the only person in the world the Joker would not kill outright. Unless he thought it would be very funny. Or it was an unavoidable part of a complex plan. Or he had to in order to kill Batman.

Whatever.

Joker happily took the duffel from Harley and kissed her on the cheek. Harley went from pleased to overjoyed in a split second. The Joker kicked everyone but Harley out and the two of them started setting up several tables. Harley laid out chemical equipment and several notebooks, while the Joker started scribbling in a new notebook about… Cassandra got out her binoculars, but she still couldn't read it. The book was tilted oddly, and what bits she could see, she couldn't read fast enough to make any sense of.

Batgirl pulled on her gas mask and got comfortable.

It was several hours later, and the Joker and Harley were well into their work when the door to the room opened and a brilliantly glowing man entered the room, floating six inches off the ground.

So "Luminus" is not just an idle name, then, Cassandra mused.

"Under normal circumstances, I would kill my henchmen for letting you in," the Joker said, "but I suppose I can't blame them for failing to keep out the Rapture. Although I never thought you'd be coming for me."

The glowing man laughed, and moved a hand to his belt. The brilliant light died away, leaving a man in a gray costume with oddly-shaped orange strips running over it. "Actually, I was only invisible."

The Joker didn't reply immediately. Cassandra watched as he flicked back and forth between "kill him now" and "he'll come in handy." Finally, he settled on "get more information." Cassandra listened closer. She had a feeling the words would be more useful this time.

"Never let it be said that we in Gotham don't hold the highest standards of hospitality. Harl? A drink for our guest," announced the Joker with grandiose gestures. But one of his gestures was a prearranged signal, for Harley's benefit. Harley reacted with a slight stiffening of the muscles in her forearms.

"Comin' right up, Mister J!" Harley announced. She headed towards the door with no intention whatsoever of getting any drinks. Instead, her entire body said something along the lines of "sneak, sneak, sneak." Cassandra had the impression she would be headed to a good vantage point, so she pulled herself farther back into darkness.

"Now, what is it that you want from me?" Joker asked.

"Actually, I think we can help each other," Luminus said. Cassandra couldn't help but smile at that. Apparently this man knew not to come to the Joker without something to offer. "You're planning something, but the police and the Batman usually try to get in your way. I need someone to keep any of the Super-crew that chases me here busy. My proposal is that I do something big and splashy—several somethings, if you want—to keep the cops and the Bat busy chasing me. They don't know my methods and may need a moment or two to regroup. On the other hand, the last time you encountered Superman, you nearly killed him. He doesn't know you, and that makes you dangerous. I propose we swap opponents."

Harley Quinn padded silently into the hallway in front of Cassandra and poked her head through the railing, to look down on the scene below.

Cassandra craned her neck to be sure she got a good view of the Joker. He was practically vibrating with savage delight. Something about Luminus' previous statement had made him very happy, but it wasn't in a way that Luminus would like. There was some other idea the Joker had in mind, but Cassandra had no idea what. Either way, Luminus would be useful to him, but not in the way Luminus thought.

"Me taking on Superman. I almost like it. It's—"

Whatever the Joker was about to say was lost as the wall on his right disintegrated. In the middle of the hole stood Supergirl. Cassandra blinked in surprise. She couldn't possibly so stupid—?

"Luminus! You're coming with me!" Supergirl announced.

Okay, maybe she could be so stupid. And her body language was all gibberish. It wasn't that she wasn't saying anything, but now Cassandra had a deeper understanding of that phrase, "It's all Greek to me." Whatever language Supergirl's body spoke, Cassandra hadn't learned it yet.

"Another Super-brat? And finally one that would look good wearing their undies over their clothes, and you don't do it," Joker announced.

"Stay out of my way," Supergirl ordered, stalking forward. Her motions were alien and unnatural. Cassandra could barely stay focused on her. Her actions were constantly at odds with what Cassandra's training told her Supergirl's body should be doing.

The unusual body language was having an effect on the three humans, though. Cassandra could see the shift and slight recoil in Joker, Harley, and Luminus as they unconsciously recognized Supergirl as something that seemed very human but was not on a fundamental and important level. The instinct to avoid her as a potential source of danger was visibly present in all their motions now.

"Oh, gee, you really do take after the Big Blue Idiot. All muscle and no brains," Joker sighed. His grandiose manner was covering up his plan again. He was waiting for Harley to make her move.

Harley had her gun out and was taking aim at Supergirl as the Kryptonian stalked into the room. Cassandra rolled her eyes. If Supergirl would take advantage of it properly, she could use the effect she had on these humans. Her unnatural presentation threw them all off balance, but now she was loosing her edge by being so open and unprotected. Cassandra prepared herself to move.

Harley fired. Joker plunged a hand deep into his pocket.

When the cap Harley fired hit Supergirl, a set of brightly colored ropes exploded from it, binding Supergirl's arms tightly to her sides. And Joker pulled his hand out of his pocket.

Supergirl fell to her knees.

The Joker opened his hand and an eerie green glow emanated from it. "Just a little something I saved from my last fight with your boss. It's no good on Batman, of course, but it keeps your Den Mother well out of my way. One tiny little lead-lined pocket, and it becomes my secret."

He kicked Supergirl over and leaned in to whisper, "You see, my dear, I've been in this game for a very long time and I'm afraid you just don't know all the tricks yet. Too bad. You're cute."

Cassandra silently laid Harley out with a single blow and gently dropped her to the floor.

"And now," the Joker announced with a grin, "you won't live long enough to learn anymore."

Cassandra tossed three smoke pellets onto the floor below and fired a grapple upwards. It grabbed on at just the perfect spot for her to swing down and snag the ropes on Supergirl as the momentum propelled her, now with cargo, out the hole in the wall.

Supergirl was still dazed, so Cassandra hoisted her over her shoulder in a fireman's carry and fired another grapple off. This one took them up and away from the Joker's hideout. They would have to pick up the trail later, because right now, getting away was the priority.

Kara awakened in a dark room with a bright light shining right into her eyes.

"What is this?" Kara asked.

"Like sunshine," a quiet voice answered.

Kara blinked, and flexed her hands. She actually did feel much better. "Thank you." She lifted her head and looked over. A girl, about her age apparently, with dark hair and brown eyes looked back. "Where am I?"

"A…safe house."

Kara looked the girl over. She was not wearing a mask, but there was no mistaking the yellow outline on her chest, or the black cape flowing from her shoulders. "Batgirl?"

"Yes. Why are you in Gotham?"

"Luminus escaped. I must bring him back to Metropolis. Why are you angered?"

Batgirl's eyes narrowed. "You nearly died. Because you were stupid."

"How was I to know he was so dangerous?" Kara demanded angrily.

"He is Joker," Batgirl said in a quiet voice. "That is all that needs to be known."

Kara glared angrily at Batgirl. "You were there."

"I was observing. You were rushing in with no plan. That is unacceptable. Now Joker will have moved and we must find him all over again," Batgirl said.

Kara opened her mouth to argue and then realized what Batgirl had just said. "What do you mean 'We must find him all over again?'"

"I can not fight the Joker alone. He is dangerous. If you don't rush in again, you would be helpful," Batgirl told her.

"Oh." Kara digested this for a moment. "Why can you not fight him alone?"

"It's Rule One."

"Rule One?"

"Batman's rules."

"Batman has rules?"

Batgirl looked at Kara like she'd grown a second head.

"My apologies. Of course Batman has rules," Kara sighed. "Actually, I should have come and found you first. It's what Kon-El told me to do. He said Gotham is different."

Batgirl frowned thoughtfully. "Perhaps. But it doesn't matter. Your actions would have been stupid no matter where you had been."

"I am attempting to apologize!" Kara protested. For the first time in a long time, she had a very strong urge to do serious physical damage to a human.

Batgirl said, very stiffly. "Apology accepted."

The two girls glared for a moment. Finally Kara said, "What do we do next?"

"We begin running down places where Joker might be hiding now. We also begin trying to determine his plan before he implements it," Batgirl said.

"What about Luminus?"

Batgirl gave a tight smile. "Police. They'll have him."

"No. Luminus is a dangerous criminal. Your police have never encountered him before," Kara protested. Having come to Gotham city for one purpose and one purpose only, she wasn't about to leave until she saw that purpose accomplished.

Batgirl turned to a computer behind her and typed for about three seconds. The screen displayed a dossier of Luminus, and his complete criminal history, and suggested methods of combating him, updated to include his presence in Gotham. "Metropolice MCU sent this to Gotham police this afternoon."

"Oh."

"We go after Joker."

"However, I did come to this city to ensure Luminus' capture. I must verify that this event will occur."

Batgirl gave her a look that said she was approving of Supergirl for the first time since they'd met. And in that instant, Kara hated her. She had been sent from her home with no family left but a tiny baby. She had arrived at her destination to discover that the baby was a man and she was now superfluous. She struggled under the weight of Batman's disapproval and Superman's expectations, and now she come across the country to a city she hated, and she had made a mistake. A big one. But now this child for some reason believed that this mistake gave her the right to approve or disapprove and that that opinion would be relevant?

Aloud, Kara said nothing. Batgirl turned her head and the moment passed.

The next night, Cassandra watched as the Gotham MCU cornered Luminus in a bookstore, after hours. Supergirl was watching from several stories up, and through two or three buildings. Before entering, the police all put on a set of goggles. Cassandra had seen these in action before. They were essentially a cross between night-vision and infrared, originally developed to aid in fights against Firefly. The goggles highlighted the differences between the solid light constructs Luminus created, and physical objects.

The fight did not last long. Luminus lost hard.

"Satisfied?" Cassandra asked.

"I am. I will continue my search for the Joker," came Supergirl's curt response. Cassandra made a face behind her mask but did not reply.

Since she had been busy finishing her patrol, she'd suggested Supergirl start scanning the city to see if she could spot the Joker and Harley. At the very least, they would discover all the places the Joker was not.

Cassandra wasn't panicking, but she was becoming concerned. Running smack into Harley Quinn by mistake was not going to happen again. And when Supergirl finished searching the city and came up empty—and she would—they would need to come up with a new strategy for finding where the Joker was.

Batman was a detective. The best in the world. Tim was a detective. Barbara was a detective. Even Dick was a detective.

Cassandra…was not a detective.

When she thought about it, most of her leads came from hitting people until they told her what she wanted to know. From reading them when she wasn't pummeling them. So unless someone who happened to know where the Joker was and was thinking something along the lines of, "I really hope Batman doesn't ask me where the Joker is" happened to walk under Cassandra's nose so she could read their body language, she was a little unsure of how to get a good solid direction for this case to materialize.

What would Batman do? Cassandra wondered. He would have noticed some crucial detail when he was in the previous hideout that would inexorably lead to his discovering the new one, naturally. Okay, what would Batman tell her to do? He always said criminals were superstitious and cowardly. But Alfred had once remarked that they tended to be on the selfish side, too. Okay, good. What would someone superstitious, cowardly, and selfish do that she might be able to use?

Cassandra pondered this as she watched the police manhandle Luminus towards their prisoner wagon. They were just about to load him inside when Montoya doubled over and vomited.

Two other cops rushed to her aid. Cassandra watched as one of the ones restraining Luminus suddenly gagged and swallowed. A third went down in the next second.

Luminus snatched his belt and was off before they could stop him. One of the officers radioed in the escape and turned back to the fallen police. Montoya and the two others were still retching, although by this time in dry heaves.

Cassandra was about to turn away and tell Supergirl the bad news when one of the officers who'd been sick giggled. Cassandra focused on them tighter, and frowned when she realized he didn't want to be laughing. It took him several moments to stop.

One of the other cops asked him why he'd laughed.

He answered that he didn't know. Maybe he'd just been a bit too edgy.

Cassandra had her doubts on that score. But on the plus side, now she had a lead. Cassandra headed straight for the precinct these cops had come from and radioed for Supergirl to join her as soon as she'd finished scanning the city.

Now it was time to see if Supergirl could follow orders.

Kara followed quietly as Batgirl snuck into the police building. She had an irrational desire to simply fly out of the city and leave this whole mess in Batgirl's hands. If she failed, then it serves her right.

As soon as Kara had joined her, Batgirl had begun snapping out orders about what they were doing next. Since she had so poorly miscalculated when she'd arrived, Kara decided not to argue with them, but that didn't make Batgirl's assumption of authority any more irritating.

However, as much as the other girl was annoying her, Kara did admire the ability to effortlessly blend in with the shadows as they were combing through the personal effects of the three sick police in a deserted locker room.

"It is safe to talk," Batgirl said.

"I know," Kara answered. "There are no heartbeats within range of the average human's ability to hear."

Batgirl turned to face her. "You count…hearts?"

"It is how I knew where everyone was when I burst into the Joker's hideout before," Kara answered. She felt, on the whole, that this simple response demonstrated admirable restraint on her part.

"Maybe it wasn't so stupid," Batgirl said slowly.

Kara raised an eyebrow and changed the subject. "Luminus escaped the police."

"You saw earlier. They will have him soon," Batgirl said.

Kara shook her head. "I give them until we finish tracking down this 'Joker.' If he is not captured by then, I will go after him myself, police or otherwise."

"You will go after no one alone in my city," Batgirl snapped.

Kara looked her up and down and finally said, "You are incapable of stopping me. Unless you are in possession of some kryptonite, as well?" Taking orders for the sake of being stealthy was one thing. She was not about to allow Batgirl to even try to prevent her from accomplishing the one thing she had come to do in this city.

Batgirl glared at her.

Kara decided to end the whole discussion. "If your police can capture Luminus, this is moot. If they cannot, I will not stay longer than it takes to see him imprisoned. Have you found something?"

Batgirl shook her head. "Whatever made those police sick, I don't think we'll find it without risking their seeing us."

"Why can we not simply ask for their assistance?" Kara asked.

"It's…Gotham," Batgirl said.

Kara rolled her eyes. Fine, she could play that game, too. "Very well. I will scan the building and tell you what I see. Do you have any suggestions on what I should focus on?"

"The three police were vomiting. Check for something they may have swallowed," Batgirl answered.

Kara nodded and began looking across the building, slowly turning to cover everything. And then she saw something. "There is an odd residue on the dispensers of that water cooler. And in the refrigerator's venting units. It's a chemical mixture that doesn't belong there."

Batgirl nodded. "We know what he's doing, now. He's poisoning police."

"Why are only three sick?" Kara wondered.

"We will ask him when we find him. Can we get those two—"

Kara raced off into the police station and removed the offending unit from the water cooler and refrigerator, making sure that both machines were not obviously malfunctioning when she was done with them and then returned to Batgirl before she'd finished her sentence.

Batgirl looked at her, apparently having a hard time deciding whether to be annoyed or amused. "Now I'll just run a few tests on them," Batgirl muttered, turning to leave. Apparently she had no intention of speaking further on her plans.

Kara narrowed her eyes as Batgirl swung away. Fine. There were other ways of getting information.

Cassandra frowned as the tests she'd run told her exactly what she'd expected. The substance on the piece from the water cooler and the fridge were both a mild version of the standard Joker toxin. It wasn't strong enough to hurt anyone seriously or permanently right off the bat, but prolonged exposure would have the same effects as usual.

Cassandra shuddered, having seen more than enough of Joker's eerie rictus-grin victims.

She was standing her safe house, mid-morning, the day after she and Supergirl had broken into the police station. They'd gotten home, Supergirl had asked for a picture of what Harley looked like without her costume and had disappeared. No further explanations. For all Cassandra knew, she was out buying pretzels.

And now she just wished Supergirl would get back so she could tell her that they might as well go after Luminus because she didn't know how to find the Joker, and she didn't think she'd come up with anything before Batman got back. And Cassandra really didn't want to go do that. At all.

"I have found him!" announced Supergirl, breezing in from the roof.

"How did you avoid the alarms?" Cassandra demanded.

"Your keypad on the roof," Supergirl shrugged. "I saw which buttons you touch most frequently and tried all the possible combinations at speed."

Cassandra stared. This security system was meant to be better than that. For Supergirl to simply be able to breeze through it was…unfair.

"It is unimportant. I have located the Joker and his…friend."

"Harley is not—what? Found them? How?"

As Cassandra watched, Supergirl's entire demeanor changed and suddenly a purely human girl, a bit waif-like, and certainly out of place in Gotham City stood before her. Supergirl opened her mouth, and in a flawless mid-western American accent said, "Miss? I'm so sorry to be asking weird questions, but I'm visiting town and going to a costume party tonight and a friend of mine told me she shops around here for costumes. She might have bought a police uniform from you? She's about yey tall with blonde hair. Do you remember her? Is this the right store? Am I lost?"

Cassandra blinked. The act would have almost fooled her. Supergirl's entire demeanor was open, inviting, just out-of-town-ish enough to garner sympathy, but not so self-conscious as to make her a target. "You have used this trick before," Cassandra finally said.

"It helps if people think you're a harmless girl," Supergirl mused.

"Police uniform?"

"They had to place the poison in the station, did they not?" Supergirl pointed out. "Someone must have done so. I do not think the Joker would have, based on your assessment of him. But that woman seemed so eager to please him…"

It was almost more than Cassandra could take. Batman was the World's Greatest Detective and Batgirl was one of his team. Not this outsider. And while Cassandra had been flailing for leads, this girl had actually been running down a real idea.

For a tiny instant, Cassandra didn't care how childish her objections were. She wanted to throw an exploding Batarang right into Supergirl's nose! It wouldn't hurt, but it would definitely wipe that smug smile off her face.

Rather than giving into her desires, Cassandra turned and picked up her mask and cape. "Where?"

Kara followed Batgirl as she led the way into the Ace Chemical Plant. She didn't understand the significance, but apparently it was somehow important. When she'd told Batgirl, the other girl's entire expression had changed. For whatever reason, they were being extra-cautious as they entered.

The building was full of catwalks, odd lighting, and old vats that had probably once held chemicals, but were now just lying around empty. Apparently the building was condemned.

Kara looked around. "Many of these walls either contain lead, have lead-based paints, or have been affected by something with lead in it. I can see through some things, but not through others. It is…confusing."

Batgirl nodded. In keeping with the plan they'd made before they'd come, Kara took off and flew off to the left, keeping low to the ground. At least this time Batgirl had had the courtesy to include her in the planning.

She spotted Harley Quinn seated on a…box in another room and so she floated in that direction, being careful to stay quiet.

She got close enough that Harley would have no time to move and then whipped around the corner and tackled the woman, rolling so that she would absorb the impact of their landing, rather than her captive. Humans were so breakable.

When Harley immediately flipped up as Kara was righting them and kicked her in the teeth hard enough to cause Kara to stagger back a few steps, she was taken more than slightly by surprise.

Then Harley glowed white and vanished.

"Luminus!" Kara yelled, suddenly growing furious.

"You're not as dumb as you look," Luminus said, sounding far too smug.

"Where's Joker?" Kara demanded.

"Not here," Luminus grinned. An imposing figure materialized in front of Kara. Batman. His dark cape swirled around his ankles in a non-existent breeze. He looked at her disapprovingly, as usual. Then he took an aggressive stance.

"Tell me where the Joker is!" Kara ordered.

Batman attacked. Kara dived out of the way, rolling. Luminus expected her to fight his pawn. What wouldn't he expect? As Kara pondered this, Batman dove at her again.

This time she grabbed the thing's wrist and flung him back at his own maker. Luminus dispelled the light-thing immediately, but Kara had followed in the its wake, so when Batman vanished, she was filling up Luminus' vision.

The look of shock that she wiped from his face was incredibly satisfying. He fell to the ground.

"Where is Joker?"

"Ask your friend. She should just about be dead, if his hyenas are as hungry as I think they are," Luminus answered with a grin.

Kara blinked. She paused to listen for a moment and zeroed in on Batgirl's heartbeat. Then she rushed through the building, barely feeling the walls she burst through to get to the other girl, who was indeed just about loose a fight with two, large, dog-like creatures.

Kara grabbed her by the back of her cape and pulled her out the back door.

"Joker is not here," Batgirl breathed.

Kara set her down gently on the nearest sidewalk. "I know. One moment."

In less than a second she had gone back into the building, snatched up Luminus by the scruff of his neck, and dragged him over to Batgirl. The building's structure had been weakened by her quick action, so she scanned it over as best she could to determine if it was empty. Finally, she turned to Luminus.

"Where. Is. Joker?" Kara demanded.

Luminus did not answer.

Batgirl pulled something out of her belt. A batarang. Its edges were very sharp-looking, and she held it with a dangerous-seeming casualness. Then Batgirl leaned in and simply looked at the man. Her mask covered her whole face, but even without using her x-ray vision, Kara could see the glare.

"He went after Gordon! He said with Batman out of town, there was no need for the cops to be unentertained. But with you two closing in, he figured he'd get the commissioner, at least. Gordon! He's gone after him."

Batgirl whipped out a pair of handcuffs and chained Luminus to one of the posts of the chain-link fence outside the Ace plant. Kara crushed the buckle to his belt and left it beside him. To where the buckle used to be, Batgirl affixed a small transmitter and pushed a button.

"Police will come soon. We must find Gordon. Follow me." Batgirl began running down the street.

Kara rolled her eyes, flew after the other girl, snatched her up by the arm and said, "Which way?"

Cassandra did not like being carried. If she'd had any kind of say in how she got to Gordon's house, this would not have been her first, third, or eighth choice. And even as they raced to get to the commissioner's house, she was desperately wanting to pry the fingers of this alien off of her arm. She could feel revulsion in every fiber of her being at the physical contact with someone who so irritated her.

Although, if she were entirely honest with herself, the actual flying was a bit fun.

Jim Gordon's house was in a residential area just across the bridges from Gotham Island. It was actually very nice with its red bricks and sprawling garden. Although the sprawl of the garden was probably due to its not being tended as often as was ideal.

In fact, the front door that hung crazily off its hinges was the only sign the two girls had that anything was wrong.

"Quiet approach?" Supergirl asked as they neared.

"No time. Go in," Cassandra replied.

Supergirl flew straight in through the open front door. Their sudden entrance startled Harley, who fell off an ottoman in the living room.

She opened her mouth wide, but Supergirl was at her side in two seconds with her hand between Harley's teeth.

"Pronounce one word, and you'll loose all your teeth," she said in a low voice that would not carry.

Cassandra was very unwillingly impressed.

"I have her," Supergirl said quietly.

Cassandra nodded and swept the first floor quietly.

She discovered the Joker and Gordon in the kitchen. Joker was about to inject a syringe of…something into the police commissioner.

Cassandra tackled him to the floor, the syringe rolling away. Gordon leaped up and bounded out of the room.

"So, he's left you to handle me on your own, eh?" Joker said with an insane grin.

"You can not beat me," Cassandra told him. She moved to a long cat stance and observed as the Joker considered his options.

"And I don't leave my people in the lurch," Gordon announced from behind him. He had retrieved his sidearm and now had the barrel leveled at the Joker's head.

The Joker and Harley Quinn were handcuffed and on their way to the police lock-up in under two minutes.

After being thanked by the commissioner, the two girls exited his house. It took some persuading, but Cassandra managed to convince Kara to stay long enough to make a report to Batman when he returned the next night.

Kara could see the party gathered on the roof of police headquarters long before they got there. Batgirl, Batman—who had apparently returned from Ireland sometime that morning, and Superman—who had come to Gotham that evening—were standing by the Bat-signal, looking at the sky.

Kara caught her cousin's eyes and thought he seemed pleased, but she wasn't sure what she was reading from Batman.

Kara landed lightly on the roof.

"Report," Batman ordered.

Kara blinked at the abruptness. Batman just gave her that look Kara despised. Kal-El did not contradict him, but did manage to look sympathetic.

"Fine!" Kara burst out, totally unable to take any more from anyone wearing a bat. "I came on far too strong when I arrived in Gotham, which nearly got me killed. Batgirl took it upon herself to lecture me, so whatever you might have to say on that score, you can save it. As of this moment, Luminus, the Joker, and…Harley Quinn are in police custody.

"As for my partner," Kara said, spitting the word, "she is uncommunicative and unhelpful. During the whole of my time here she told me very little but expected me to trust her anyway. If she knew anything about the case, I was fortunate to find it out." She turned to Batgirl. "Honestly, I hope never to see you again in my life!"

"She is undisciplined and stupid!" Batgirl said, turning to Batman. "She first rushes in without stopping to think and then runs down leads without consulting me! How am I meant to work with someone who's methods are so erratic!"

"Stubborn hypocrite!"

"Arrogant fool!"

"Enough!" Batman said.

"Not nearly!" Kara snapped back at him. To her immense gratification, he actually blinked. "I've had it with you!" She pointed at Batgirl. "I've had it with this city!" She marched up to Batman. "And I've had it with being intimidated by people wearing this!" She poked the bat on his chest, lightly so that it wouldn't cause him pain. "I'm leaving! And I hope I'll never, ever need to come back!"

Kara took off, furious, and totally unwilling to take another moment in Batgirl's presence. If she never saw another human wearing that stupid bat-shield in her life, it could not be more welcome.

Behind her, she vaguely heard Kal-El make some excuses before he followed, but by that point she was high enough to go faster than the sounds. She rushed away, towards a tiny farm in Kansas where people were happy to see her.

If an observer had watched their conversation, they would have noted that Batman was aware of Superman's presence before Superman said, "Hello, Bruce."

"Clark." Batman turned from the Bat-computer to look at his guest. Although his cowl was pushed back from his face, he was otherwise in full costume.

"I've had a talk with Kara. There won't be another outburst."

"She had good reason. Batgirl knows she is a less than ideal partner."

Superman smiled. "So Kara told me. Before she admitted she might have been a bit difficult herself."

Batman's mouth twitched.

"It's a shame, actually," Superman sighed.

"What is?"

"They actually make a good team. Luminus was barely even a blip on their radar, and they completely circumvented all but the most benign effects of the Joker's plan. According to the newswire, he didn't actually manage to kill anyone before he was back in police custody," Superman observed. "Kara sometimes forgets to think things through, but she's quite clever when she does. Batgirl made her stop and think."

"Kara is also resourceful when it comes to getting information, which is something Batgirl hasn't quite got the knack of. They are a good team," Batman observed.

"Yep. Too bad."

Batman offered a slight smile. "Agreed. We should probably avoid contingencies that involve their working together in the future, however good at it they might be."

"Indeed. Tell Alfred I said hello."

"I will. Give Lois my best."

THE END

Author's Notes: A little extra notes, in case anyone was wondering. If you weren't, feel free to skip these.
I thought this story up because Cassandra and Kara seem to me to be like oil and water. No matter how often they are in one another's presence, they're never going to mix. I also wanted to get this universe established in my head because I like some of the elements of the comics, but I'm more a DCAU fan at heart so I wanted to firm up where I actually draw my inspirations from. This way, if I go to writing these stories in the future, I'll know before I begin.
What I've tried to do with Kara in this story is to make her a lot less easily compatible with Earth generally and the US specifically than she was in the Superman/Batman intro she got. Because as much as she might have gotten a bright, shiny day to announce herself to everyone, she's from an alien planet, she's grieving for loosing her family, for her friend that died, and she was turned evil by Darkseid. Her managing the bright, shiny day as well as she did was pushing it. I will own to drawing heavily on Seven of Nine for inspiration here.
Cassandra mentions "Batman's Rules" and that rule one is no one fights the Joker alone. That rule was taken from Lacey McBain, in the "Comrades in Arms" series. In case it came up for whatever reason, I did create ten more Bat-rules. The rest have to do with general Bat-preparedness for fieldwork, but they're not relevant enough to be included in the story, so I'm really just showing off at this point. :)

Bat-Clan Rules:

1. No one takes on the Joker alone. Ever. Not even Batman.

2. Codenames are to be used at all times in the field. Even in completely secure settings.

3. Field-work requires heavy hydration. Whenever possible, drink fluids often.

4. Check-ins are at the top and bottom of every hour. If, for any reason, you are aware that you will miss a check-in, make every effort to notify someone before this happens. If anyone misses three check-ins in a row without notification, they are to be presumed missing and actions to locate them are to begin immediately.

5. Batarangs, smoke bombs, grapple lines and guns, lock-picking equipment, fingerprinting kit, camera, flashlight, luminol, radio, panic button, map, and notebook or recorder are ALL required for field-work. Required equipment for contingencies to do with specific suspects will be listed under that person's "General Orders" (e.g. Poison Ivy – General Orders).

6. All equipment and costume parts are to be kept in the cave or in a secure location at all times, even when they are undergoing maintenance.

7. No equipment is ever to be touched with bare hands, nor is any hair to remain on costumes.

8. All safety checklists for all equipment must be completed both before and after that equipment is used in the field. This includes costumes. Any damage must be logged. Repairs must be completed before the equipment is returned to field use.

9. Non-consumable equipment (such as Batarangs and grapples) should be recovered whenever possible. Log the number taken out and the number returned.

10. All logs, including PT logs, are to be submitted for review. Any comments are to be added in the appropriate sections, not at random points in the log entry.

11. Open case files are to be logged for common access. All open cases should be reviewed by all parties at least once per week. Any evidence or forensic work must be appended or linked to the appropriate file. Any relevant comments to any open case are to be added IMMEDIATELY and flagged.