Keep "Magic" by The Cars in your queue. We're going into Loki's PoV now and backtracking a bit.

The second time Loki heard a noise coming from that corner, he began to doubt that shoddy mortal building materials were to blame. The third time, he was certain. It wasn't the sound of creaking wood and plaster, it was the slightest creak of boots against flooring and a gasp that hadn't been fully stifled. It was a good thing Thor had developed enough prudence to use the nameless tongue when discussing sensitive matters. A pity his human friends couldn't do the same. They were being watched by an unseen observer.

Well, two could play at that game.

Loki cloaked himself and left a projection in his seat. Immediately, a strange, synthetic sort of noise blared from right behind him, and he nearly spoiled his ruse by knocking his chair over on the way out of it. "What the Hel?"

A cloaking spell, done properly, did more than hide the subject from sight; it also prevented them from making any sound that could give them away. It did not, however, silence those sounds entirely; the cloaked individual could still hear themselves perfectly well. Loki didn't particularly need help keeping his steps silent, but he enjoyed having the freedom to make unheard commentary about what he observed when he felt like it.

He had never heard the sounds now emitting from him, let alone made them himself. Being invisible even to his own eyes, however, his ability to investigate the source was limited. He felt around as far as he could reach on the back of his shoulder, and his fingers found a small object that certainly wasn't part of his armor. It was made of metal and vibrated slightly in time to the sound coming from it. Irritation turned to amusement as he realized what had happened. "Well done, Stark," he muttered. The man must've stuck the thing to his clothing when he patted his shoulder. Had the device automatically activated because Loki had cast a spell? Had Stark managed to develop a method of detecting Loki's seidr using mortal technology, and in so short a time? He had clearly underestimated him.

By the time Loki came up with this hypothesis, the initial strange sounds had become a distinctive beat played on instruments he couldn't identify, and then a male voice began singing.

"Summer, it turns me upside down

Summer, summer, summer

It's like a merry-go-round."

Loki did his best to ignore the song and do what he had actually set out to do by casting the cloaking spell. He could have simply banished the musical device into his dimensional pocket, but it was a worthy prank (considering it had been done by a mortal), and it deserved Loki's respect.

The projection remained at the table to participate in the conversation, which had turned towards the matter of Hydra, while Loki himself crept closer to the corner whence came the suspicious noises. Whoever was lurking there was completely invisible. Loki wondered how such a thing could be accomplished without seidr, but that mystery could wait.

"I see you under the midnight

All shackles and bows

The high shoes with the cleats a-clickin'

A temperamental glow.

How could you let me go?"

Loki couldn't make any sense of the words in this song. Had Stark chosen it for a reason? If so, it was not yet apparent. He shook himself. He had a job to do. He moved closer to the corner, eyes narrowed. For the briefest moment, he saw a flicker—the merest outline of a person, barely visible and with the colors stretched as though seen through a prism.

He reached for where he'd seen an arm. There was definitely something there, but instead of touching a solid surface, it was like he'd tried to grab a fistful of sand. After a brief moment of resistance, his fingers passed through. Unfortunately, the attempt hadn't gone unnoticed by the phantom eavesdropper. A much more solid force struck his chest, followed by a faint, feminine grunt of surprise (or perhaps pain). His opponent wasn't entirely unprepared for an invisible attacker, then. Just unprepared for one made of sterner stuff than mortal bodies.

He tried to grab her again, only for his fingers to pass through her arm a second time. Perhaps if he could grab just when she was trying to strike him, he'd be able to gain purchase.

"Oh, I got a hold on you

Got a hold on you

I've got a hold on you tonight"

Well that was just ridiculous. Whatever had made Stark choose this song, it was perhaps more fitting than he had imagined. Not only was she invisible and partially incorporeal, she was also quite skilled in combat, and proving exceedingly hard to pin down.

"Oh, I got a hold on you

I got a hold on you

(Got a hold on you)

That's right"

The music was mocking him. No matter what he tried, he simply couldn't get a proper grip on the phantom. After yet another failed attempt, he felt a hand on his chest. The fingers pushed through, and the sensation was decidedly unpleasant. Loki suspected this was a strategy she was very familiar with. If she could control how solid she was at any given moment, then that would make killing an opponent a simple matter of reaching into their chest cavity and scrambling things about. Had he been a mortal, she might have already succeeded in killing him with this maneuver, but the density of his flesh seemed to be giving her some trouble.

Still, he wasn't keen on letting her continue. She was going to regret trying this on a Jotun. He untied the little knot of seidr and let his true form emerge. He didn't put any extra effort into stealing the heat out of her hand, but he didn't really need to, given its position. There was a small yelp as she wrenched her fingers free, and her image flickered again. Resuming his Aesir form with a satisfied smirk, Loki dropped low and swept a leg across the space where the woman's feet were. Though she didn't fall when his leg passed through hers, she did stumble.

"Uh oh, it's magic

When I'm with you!"

Ah. That was why he'd chosen this song, then. Delightful. Loki decided it was time to put his entire focus into this struggle. He let his projection reply to what Romanoff said before dispelling it. A moment later, he dropped the cloaking spell too. His brother and the mortals all reacted dramatically, but he couldn't enjoy that just yet.

"Uh oh, it's magic!

Just a little magic

You know it's true

I got a hold on you"

As Loki had hoped, the phantom took advantage of his new visibility to attempt to strike him again. The instant the blow connected, he caught her by the wrist. This time, his grip held. Not wasting the advantage, he turned and flung her onto the table, where she finally flickered fully into sight, like three images appearing and blurring into one.

"Oh, twisted

Under, sideways, down"

Her appearance was greeted by much startled shouting from the Avengers—all except for Barton, whose eyes were wide. Before the phantom could recover from the stunning effects of colliding with the table, Loki reached for the odd mask and tore it away, unspooling long brown hair and revealing a face not quite as dark as Brunnhilde's, with striking features and large blue eyes. She gritted her teeth, and her lower half sank through the surface of the table so that she was standing inside it, her guard up, surrounded by the seven of them.

"I know you're getting twisted

And you can't calm down."

Loki decided Stark's prank had run its course and let the device fall into his dimensional pocket, putting an end to the music.

"Agent Starr?" said Barton. "What are you doing here?"

That brought the others up short. The phantom woman hesitated as well.

"This woman is one of your shield-sisters?" said Thor, lowering his umbrella an inch or so and relaxing his position in front of Banner somewhat. Not a SHIELD agent he was acquainted with, then. Interesting. Just how large was the organization?

"Yeah, she's with SHIELD," said Barton. His eyes narrowed. "Unless she's with Hydra. I've worked with her on a couple of missions."

"I'm not with bloody Hydra," she snarled.

"Did Fury send you to report on us?" said Romanoff.

"Fury isn't the one who sent me," she said, her eyes on Barton. "Is it true? What you said before. Is Pierce one of them?"

He nodded. "He's their leader."

Her eyes flashed. There was a deep pain there, as well as rage and unmistakable betrayal. It was strange to see in so young a face. Before any of them could move or say another word, she vanished from sight again. A second later, the mask Loki had removed disappeared too.

"How many agents like that does SHIELD have?" said Rogers, while Loki and Romanoff immediately began searching for a trace of her.

"As far as I know, she's one of a kind," said Barton. "Some kind of freak accident when she was a kid."

Neither Loki nor Romanoff found any sign of Agent Starr in the room.

"Heimdall, can you see where she went?" said Thor, which earned him quizzical looks from Stark and Rogers. After a moment, he glanced at Loki and shook his head.

"Impressive," said Loki.

The young maiden who'd brought the drinks now strewn about the floor appeared in the doorway bearing a large platter laden with plates of steaming food and stopped short, mouth falling open.

"Uh, could we get some towels please?" said Banner. "We had a little accident."

Extremely confused, she quickly set out their plates and then hurried off to get the towels.

"So...that was weird," said Stark lightly. "Should we be worried?"

"Leave her to me," said Barton.

"You're not leaving, are you?" said Thor.

"I doubt she's on her way to warn Pierce that we know about Hydra, but Fury needs to know Pierce is using assets like her to keep tabs on us," he said. "Nat can take it from here."

Thor looked rather upset, but didn't protest as Barton left the room. "He didn't even get to try the shawarma," he said sorrowfully. Loki was torn between amusement and pity. Thor had wanted so much more from this meeting than just a few minutes with all of them together.

"Why was The Cars playing just now?" said Banner. "That wasn't coming from the restaurant's speakers."

Loki popped the device back out of his dimensional pocket and tossed it to Stark. "Your revenge was quite well executed," he said.

"Thanks," said Stark, clearly downplaying how pleased he was. Loki was going to need to come up with something truly special for his next volley in their contest.

"Is that what music sounds like now?" said Rogers.

"More like a quarter century ago," said Stark. "Don't worry; we'll get you caught up on all the rock and roll you missed."

X

Ava could scream. Bill had convinced her that SHIELD deserved her trust, that they were the only ones with the resources to help her. And yet all this time, they were under the control of Hydra. Did Bill know? No, she refused to believe that. He was good and kind, and they'd fooled him like they'd fooled her.

She thought of all the tests they'd done on her as a girl, all the missions they'd sent her out on starting before she was old enough to legally hold a job. She'd tolerated years of assignments on the idea that at least she was serving some noble purpose while she waited for them to keep their promise. But even that wasn't true. She'd sold her soul for nothing. She'd just been a cog in a corrupt machine that didn't care if her cells were ripping themselves apart every day as long it didn't stop her from getting results.

By flitting from one empty backseat to another across a number of vehicles, she was able to get a lift back to the Triskelion in minutes. Staying fully invisible, she stalked past unwitting agents and staff, through walls and doors, and up many flights of stairs. At the top floor, she went through one final door and stopped. Alexander Pierce was sitting behind his desk, in the middle of a phone call.

"I'm aware of that, Senator," he said.

Ava focused on being present in the room, grimacing beneath the mask at the sensation. Pierce froze, eyes on her. "Let me call you back," he said, and set the handset back in its cradle. "Agent Starr. I wasn't expecting a report from you until tomorrow."

"I've heard all I need to." She pulled off the mask and dropped it on his desk.

"How concerned do we need to be about Asgard?"

"Loki made me in about five minutes, but that's not why I'm here." She watched him closely for his reaction. Whatever lie he came up with, she wanted to see the truth in his eyes. "It's been a decade, you know. How much longer do I have to wait for SHIELD to fix this?" She raised a hand, the image of which was spread across three positions a few inches apart. They flickered and came back together. What wasn't visible was the tearing sensation that accompanied the separation and reintegration.

If she'd blinked, she would have missed the flash of irritation on his face before he hid it with concern. "With Dr. Pym and Dr. Foster both gone from the agency, it's not so easy, but we've made progress."

"Then why not bring them on? They worked for SHIELD before. You know Bill's like a father to me. He'd do it in a heartbeat if he had your resources."

"When he left SHIELD, he made it very clear he wouldn't be coming back," said Pierce.

More like he was too clever not to notice whatever Hydra was up to the longer he stuck around, she thought. "What about the stacks of research and hypotheses on molecular disequilibrium he's sent over since then? Has anyone looked at those?"

The veneer over Pierce's frustration was growing thinner. "What happened to you and your parents in Argentina was a tragedy, Agent Starr," he said, getting to his feet and starting to walk around the desk. "It's a miracle you survived, and you've been able to do so much good since then. We recognize that, and we're using every resource we have to ensure that you can live a normal, healthy life." He laid a hand on her shoulder, slowly and carefully so that he'd be able to make contact. Ava had never hated anyone more. "But this is a one-of-a-kind condition, and curing it is a steep, uphill battle."

She had heard versions of that speech several times over the last few years. She wasn't going to smile and nod and run along on her next mission this time. "Maybe it is," she said. "Or maybe you're just stalling because you don't want to cure your only subject before you can safely bottle up what I can do for your Hydra agents."

Pierce's eyes went very round. He dove a hand inside his waistcoat pocket, but Ava dove one of hers straight through his chest before he could retrieve whatever he was after. He froze, his whole body tensing up. She found his heart and let her fingers loosely brush it. It was beating incredibly fast.

"I want to see both of your hands," she said through her teeth. "Slowly."

"Agent Starr," he said as he raised them. "Think very carefully about what you want to do here."

"I've already done that, sir," she said.

"Hydra can offer you so much more than SHIELD has," he said.

"Oh really? Can it make up for ten years?" She dove her left hand through his shoulder and dragged her fingers down through the sinew and bone of his arm. It didn't feel remotely good for her (though much better than when she'd tried it on Loki), but Pierce cried out in pain and tried to back away. She stayed on him all the way to the wall of windows. "That's what it's been like. Every day." She swiped down his side, and he screamed, barely staying on his feet. "Every hour."

The third swipe was to his stomach, at which he crumpled to the floor, gasping for breath, staring up at her in horror. "You missed your chance to make me an offer a long time ago. I hope Tony Stark and Captain America and the bloody Asgardians send all of Hydra to hell. SHIELD can go with it for all I care. But you'll already be there." With a yell of rage, she forced her right hand to nearly full solidity and ripped it back out of his chest.

If any of you still thought this was going to be one of those fics that awkwardly squishes an extra character into the story without letting it change organically around them...NOPE! This way is much more fun. Muahaha. I figure if Ava wouldn't hesitate to use Cassie as leverage against Scott, then she wouldn't hesitate to kill the head of the organization that used her and lied to her for years, letting her think she was working for the good guys the whole time.

Backtracking to Tony's prank, though. This is what my brother helped me figure out last chapter. It occurred to me that Tony's brilliant prank device was going to ruin Loki's attempt to catch Ava, and I just stopped writing for a while, totally stumped. I didn't want to give up the prank, and I refused to give up Loki catching Ava. What to do? Well, when I lamented this to my brother, he immediately suggested that Loki's cloaking spell should cover sounds coming from him too. Perfect. That way he could enjoy the song while he did his thing, and then everyone else would start hearing it when he reappeared.

It's come up a few times before in the fic in very minor ways, but I'm going with the comics thing where Aesir and Jotnar have triple the body density of humans. I thought that would interact with Ava's phasing in interesting ways.