As it says in the summary, this story is tied to a previous story called "Unforgivable". You don't have to read it, though.

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Three months have passed since the Daxamite Invasion in National City. There was so much damage caused that day and lives were lost. Luckily, the citizens of National City can always look up to Supergirl to keep their city safe in the darkest times. As long as they see her flying in the skies, they know they're safe.

Life has been okay for Supergirl since the invasion. The only downside is that she had to say goodbye to Mon-El, who has been a good friend to her. There was a time when she was in love with him, but her feelings for someone else kept her feelings for Mon-El from growing. Her mind then shifted to the person that she was truly in love with. She wanted badly to use her interdimensional extrapolator and visit him, but decided against it.

She just went about her usual business, flying through the night sky. It was quiet in the city tonight, which was okay since stopping criminals isn't the only reason she soars through the clouds every day. Flying always helps her calm down and stay relaxed.

However, her relaxing flight comes to an end when she noticed someone on the roof of a building. She couldn't make it out from the distance, but she could tell it was a man in black. He entered through a ventilation shaft. She followed.

She enters the building, capturing plenty of attention.

"Supergirl, what are you doing here?" asked one woman.

"Someone's breaking in," said Supergirl.

The people started to panic.

"Calm down," said Supergirl. "Just act natural. I'll keep an eye out."

The people did as Supergirl said and went about their business like it's a normal day. Supergirl checks room to room for the intruder. She even uses her x-ray vision and super hearing, although she did accidentally stumble upon some "shenanigans" in a closet, which made her want to gag.

"Focus, Supergirl," she said to herself.

She kept on listening, until she heard something. It sounded like someone bumped into a wall, really hard.

She takes the elevator downstairs to the floor where she heard the commotion. It came from the janitor's closet. She listened in carefully.

"Where is it?" A deep, modulated voice boomed.

"Where is what?" replied a scared man.

"The Mother Box!" yelled the modulated voice.

"I-I don't know," said the man.

"Unless you want to be able to hold a mop again, you'll tell me who you gave it to!" threatened the modulate voice.

Supergirl opens the janitor's closet. She finds a green-skinned man wearing a janitor's outfit and a man dressed in black with a bat symbol on his chest. The latter is what surprised her the most.

"You're... you're... you're him," she said, surprised. "What are you doing to this man?"

"This is none of your business," growled the man in black.

"Threatening to break an innocent man's arm is my business," said Supergirl, putting emphasis on "is".

The man in black takes out a smoke capsule and detonates it, covering the area with smoke. Supergirl manages to clear the smoke with her super breath, but the man in black was gone.

She looks at the alien janitor and says, "When this is all over, I'm coming back for you."

"Better you than that freak," said the janitor.

On the rooftop, the man in black was about to jump off the roof until Supergirl showed up. She tried peeking into his mask, but for some reason, she couldn't.

"We're not done," said Supergirl.

"You don't know who you're dealing with," said the man in black.

"I think I kinda do," said Supergirl. "You're the Batman, the vigilante from Gotham City. You knew my cousin, Superman."

"He never knows how to keep his mouth shut," said Batman.

"Last chance. Are you gonna come in quietly or do I have to force you?" said Supergirl.

Batman throws a metal projectile shaped like a bat at Supergirl. It pierces her shoulder. Shocked by how it hurt, she pulls it out and noticed something. The projectile was tipped with Kryptonite. She tosses it away and turns her attention back to Batman, only to see that he wasn't where he was a few seconds ago.

Supergirl lands on the roof and keeps an eye out for Batman. She finds him hiding behind the fans. She flies forward and tackles him, slamming him to the edge of the roof.

"If you know my cousin, then you know I have x-ray vision. There's nowhere you can hide," said Supergirl.

"I'm not trying to," said Batman.

Batman takes out three silver spheres and throws them at Supergirl. They attach themselves to her body and started shocking her. Fighting the pain, Supergirl rips the spheres off herself and throws them away. She sees Batman about to jump off the roof. She uses her heat vision and knocks him down.

"What are you doing here anyway?" asked Supergirl. "We already have a vigilante in a mask to beat criminals to a pulp."

"I'm here on personal business," said Batman. "Business that doesn't involve you."

"If that business involves hurting innocent people, then I'm gonna have to get involved," said Supergirl.

"Innocent? You have no idea what's going on," said Batman. "Regardless, you're not taking me in."

"Have you ever fought a Kryptonian before?" asked Supergirl.

"Apparently, your cousin never told you about that one time," said Batman.

Supergirl approaches Batman and started throwing punches, but Batman was quick, dodging every punch. Batman blocks a few punches, until Supergirl speeds up her punches to finally punch him in the face. She throws a couple more punches before spin-kicking him in the stomach, knocking him down. Batman gets back up, blocks a punch, and punches back, which barely affects Supergirl.

"Did your mother ever teach you that it's not nice to hit a girl?" asked Supergirl.

"As a matter of fact, yes," said Batman. "But I'm willing to make an exception when necessary."

Batman detonates another smoke bomb and disappears into it. Supergirl zooms into the smoke and comes out the other end, but she couldn't see Batman. She heard a gun click. She turns around and sees Batman with a makeshift gun. He fires, but she catches the round. The round explodes and releases Kryptonite gas. She struggled to breathe as she feels herself getting weaker by the second.

"You're strong, you're skilled, but not very smart," said Batman.

Batman takes out a ring made of Kryptonite and puts it on. Supergirl gets up and throws another punch, but Batman dodges it and punches her with the Kryptonite ring, knocking her down. Supergirl lies on the ground unconscious, which gives Batman the opportunity to escape.

Later that evening, Supergirl wakes up in the medical bay in the D.E.O., surrounded by James, Winn, Alex, J'onn, and M'gann.

"What happened? How did I get here?" asked Supergirl.

"We tracked you on top of a building. You weren't moving, so J'onn went to check on you. He found you lying down, brought you here," answered Alex.

"You're lungs were filled with Krpytonite and the bruise on your face had traces of Kryptonite," said J'onn. "Don't worry. We took care of it. You should be fine by tomorrow morning. I suggest you rest for the evening."

"Who did this to you?" asked James.

"It was... the Batman," answered Supergirl.

"The Batman? As in 'the' Batman? Vigilante of Gotham City Batman?" asked Winn, surprised.

"Yes, that Batman," said Supergirl sarcastically.

"And he... beat you?" questioned Winn, raising an eyebrow.

"He had Kryptonite grenade launcher and a Kryptonite ring," said Supergirl.

"Did you sneak a peek under the mask?" asked Alex.

"No. His mask was lined with lead," said Supergirl.

"What was he doing here in National City?" asked J'onn.

"He was interrogating an alien janitor for something called... the Mother Box," answered Supergirl.

J'onn puts on a surprised look on his face at the sound of the two words Supergirl just said.

"You know what it is?" asked Supergirl.

J'onn turns to Alex and says, "Alex, send some agents to that building and find that alien."

"On it," said Alex.

"Sounds like it's serious," said Supergirl.

"It's very serious," said J'onn. "I'll explain it tomorrow. Right now, rest."

If you read Chapter 68 of "Unforgivable", then you'd understand why the Mother Box is included here.