Steve's brain caught up with his fight or flight response and firmly told it to "sit down, soldier." The kitchen seemed to pass into breathless stasis for a beat. He had time to notice that while Loki always seemed fragile next to his brother – a cheetah next to a lion, a grey heron next to a swan – this time she really looked like he could break her by accident, just by shouting too loud.

Loki's female form carried over from her male one the slender elegance, the thin wrists and fingers, the fine bones. With her wide green eyes and pointed chin, all the refined lines of her face, she had an elfin look, delicate and fey. Altogether unsuitable for going up against the solid wall of muscle that was the god of Thunder. Just for a little while, as everything teetered on the lip of the future, Steve thought that even Thor would see it, would remember that you don't get to be the good guys by beating up on defenceless women.

But then Loki raised her chin, her mouth tipping up at the edges into a gleaming razor-blade of a smile. "Brother." And Thor's look of increasingly horrified shock flipped over into outrage.

"You..." he gave a bellow of fury and charged. Steve leapt up, put himself between Thor and Loki and was flung aside with one casual backswing of Thor's left hand. He flailed through a collection of chairs, knocking Natasha off her perch, both of them smacking together into the wall by the door, rolling and scrambling up together to draw in identical breaths at the sight of the brothers' touching reunion.

Thor had Loki by the neck, had picked her off the floor and was throttling her with one hand, the other fending off her attempts to claw his eyes out with her long nails. "What is this?" he shouted, and shook her like a puppet, "Do you have no care for your family at all? It is not enough for you to shame us all with your madness, but you must now parade your perversity in front of all nine realms?"

Steve looked around. The other Avengers appeared uncomfortable at this, shifting in their spots, avoiding each other's eyes. But Natasha had taken her seat again with an air of "not my problem," and no one would look at Steve. Well, damn them all.

He picked up one of the kitchen chairs and smashed it over Thor's back. "You let my wife go, Thor. So help me, put her down right now!"

But Thor never looked away from Loki's face. She had managed to get him to loosen his grip enough for her to breathe and she was now laughing like a hyena, with tears streaming down her cheeks. "A parade! Dear brother what a marvellous thought. A parade for the Captain's wedding, with all of Midgard watching."

"We'll do it," Steve said, despite the fact that he hadn't actually been asked. But Loki didn't spare him a glance. It was pretty clear that for both of the Asgardians the rest of the universe had ceased to exist, and only they were left, wrapped together in tight, incestuous bonds of hatred and affection that had lasted a thousand years and would last a million more.

And it was pretty sick, wasn't it, to feel suddenly jealous that Loki didn't love him with the same intensity with which she hated her brother, but Steve put that down as a regret for another time as he seized the still-hot pancake pan from the stove and brought it down like an axe, edge-wise, on Thor's wrist.

Thor let go. Loki fell to her knees, supporting herself with one hand, while with the other she wiped her streaming eyes, her head bent and her inky hair fallen forwards to conceal her face. And Steve, Steve had seen this somewhere before. He went chill as though he were just waking up again from the Arctic ice, and then red hot, put his shoulder down and charged Thor, ramming him hard, forcing him to take three steps back and clatter to a stop against the table. "No! You don't touch her. You don't touch her ever again!"

Tony and Clint were at his elbows, Tony trying to prize him off Thor, Clint getting in between them, holding up both hands, palms out for peace.

"Thor?" her voice was rough, soft as a cat's paw, a sweet little alto awash with poison. She lifted her head with a very familiar defiance, bright and hard and so damn brave. They called her a coward in Asgard. Steve never could understand it.

"Do you remember what you did to me the last time we were in this place together, brother? Do you remember what I did afterwards, to repay you?"

Thor shuddered as though his righteous anger had taken the bone marrow out of him when it departed. There was a pair of matching cracks as his clutching hands broke the edge of the table. "You will not speak of that here!"

"Oh, will I not?" but this expression reminded Steve of her confession of what she'd like to do if she had Thor and a sharp knife in the same room. He remembered, belatedly, that the reason Thor was still on the team at all was that Loki could wipe the floor with all the other avengers, separately or combined. Just as they needed Thor to stop Loki, Loki was the only one in the room with even a half a chance of bringing Thor down.

Getting to her feet, she smoothed down her clothes and tossed her hair, setting all the ornaments ajingle. "Yet you seemed so eager to discuss my perversity. Did I ever tell you that I recorded the entire incident on several surveillance devices?"

Thor made an inarticulate noise of protest and actually backed up, taking the table with him. Loki followed him, stalking him – beautiful and cruel as ice. "As you so kindly point out, another depravity on my part will hardly raise an eyebrow at this stage. One can only sink so low. But you, my dearest brother? Imagine the reaction if your shame were broadcast through every Midgardian TV screen, and through the scrying devices of every sorcerer in the nine realms. The crown prince of Asgard? Odin's precious favourite son? Why, Father would never be able to show his face in public again, and he so old and tired already. It would crush him."

And this was why Tony had been right, years ago, to say Loki's superpower was not his magic at all, but his words. As Steve watched the power shift between the two gods, sensed Thor recoil, as though stabbed through the gut, he had the time to wonder if this was why Thor always attacked first – because if Loki was allowed to speak at all, the damage was already done.

"You vile, treacherous woman. You mare in heat—" Thor's retreat halted as his temper flared again. It took Clint, Steve and Tony all together, braced as though trying to hold back a tank, to stop him from running in and getting his hands back around his sister's throat.

Loki didn't flinch. "You put this weapon into play between us, Thor," she sneered. "Do not complain now when it comes more apt to my hand than to yours. Now, look around you. None of your little friends knows what I'm talking about. Shall I explain it to them?"

"No!" The onrushing wall of force faltered as Thor looked hard at his sister and slumped slightly, resignedly at what he saw. His right hand clenched around Mjolnir, but his left rose to cover his eyes and rub the tension out of his forehead. Tiny stinging sparks of electricity crowded every surface in the room and snapped and stung when anyone moved, and Steve rubbed his fingertips together with a rising sense of wonder. Was this... victory? In all their thousands of rounds of enmity, in every real battle they'd ever fought, Loki had lost. Surely this wasn't the one she would actually win?

For a moment longer, stubbornly, Thor contemplated a future in which everyone knew he was as argr as his sister. And then he sighed. "No, brother. Please. Do not do this to me, or to our family. Tell me what you want of me instead and I will give it to you, if I may."

Loki's expression was suffused with bewilderment, as though she couldn't believe she'd won either – as though she had long accepted that her lot in the universe was to be defeated over and over again, and she could not quite grasp what to do with success.

"Don't go signing away the Earth," Stark warned in the moment of suspense. "It's not yours to bargain with."

"He will want the throne," Thor said, looking at Tony with an expression of such self-doubt, such loss that Steve almost felt sorry for him. "That's right, Loki, isn't it? You want the throne of Asgard. You always have."

"I do not." In the braced and stunned silence of the sunny kitchen, with plates scattered on the floor and surrounded by half dressed Avengers, Loki moved to slip her hand into Steve's, allowing him, alone out of all of them, to feel how she trembled. "I want you to swear, Thor, that you will let no one, no one in all the nine realms, kill, ensorcel or otherwise destroy Steve Rogers. That you will protect him and his children and avenge him if you fail."

Disbelief on the faces of his friends, and Thor looked half reprieved half mystified. "This I would do of my own will. You did not need to—"

"Swear it!"

Thor broke out in a wide sunny grin and swore it. Steve used the frail hand in his to pull Loki to him and enfold her, his chest to her back, his chin resting on the top of her head, feeling the faint and hidden vibration of her anxiety through his skin. Oh, honey. He was worried for her – he really was – or he would have been if it were not for the fact that a great glossy bubble of joy was shouldering its way up out of his confusion and pushing everything else aside.

Because victory was so rare a thing for her she scarcely recognised it when it came, and still she'd handed this one entirely to him. Because she'd beaten Thor and had given up the chance of humiliating and torturing him until he wept. And she'd done that because she loved Steve more than she hated her brother.

"Sweetheart," he said, not giving a damn that Tony made a little gagging gesture at the word, "I don't need your protection either."

Her sideways smile was all for him, wicked, smug and proud. "Yet you shall have it, nevertheless."