No real spoilers, only my own take on what I'd like to see, which I'm sure we won't. Some medical details handwaved to stick with canon, which will undoubtedly ignore pesky details like frostbite, etc. Special thanks to DFMB for once again coming through with the Insta!Beta on short notice, especially Cartographical for the comma patrol (-_o).
Without consciousness to spur the fight, her breathing slows to the point where he's not even certain she's breathing anymore. Castle thanks whatever inspiration made him send Derrick Storm off to the Himalyas that once, so that at least he knows what not to do. Don't rub her arms, don't move her too much, just tilt her head gently back, cover her mouth with his and breathe warm air into her lungs.
He's stopped shivering himself, and he knows that isn't good. His own sudden conviction that he's burning up can't help her; there's no actual warmth to share, just an illusion brought on by his own body beginning to shut down. He can't do that, can't go to sleep no matter how warm and peaceful that choice might seem. Kate isn't dead until she's warm and dead; it's up to him to keep her alive until help comes. Castle breathes in for himself, out for her, and hopes that help has figured out they need it, because if help doesn't come soon they might stay like this forever, frozen in the embrace they never really had while they were alive.
'Let her go, bro,' he hears Esposito saying, and he smiles because it's warm where he is, and Kate is there, magnificent in a tiny red dress, endless legs peeking out from a slit that goes all the way to her waist. She's barefoot, carrying her shoes, and he realises there's sand beneath their feet. It's a beach, and Kate is holding out her free hand, her smile a mix of shy and lascivious that leaves him frozen in awe.
'Castle, listen to me, you gotta let her go,' Esposito is saying, and suddenly the beach is gone and the world is blue, and Esposito is kneeling in front of him, his face almost purple in the light reflecting off the frost on the walls. Ryan is standing above him, gesturing and shouting at someone outside the door, and the door is open. Help has finally come.
'Kate.' Castle can't actually feel his body at all; it takes all his concentration just to tilt his head enough to look down. Kate is still curled against him, her head tipped back against his shoulder, frost on her lashes and in her hair, her face slack and grey.
'Let the medics take her, Castle.' Esposito's voice breaks, and when Castle raises his eyes again, either ice is forming on the other man's face or he's crying. 'Please.'
'Kate?' It's not a question any of them can answer, not while Castle's holding her, but he can't - physically can not - let go. His arms, his legs, his whole body feels as if it will never move again, as if he'll be shaped around her absence long after she's gone.
Strangers come to replace Esposito and Ryan, and one of them pulls Castle's eyelid back to flash a light in his eye. 'Do you know how long you've been in here, sir?'
Castle closes his eyes, unable to find words. How the hell is he supposed to know the answer to something like that?
'What's her name?' asks the other, and that at least much Castle can manage. It seems to be the only word he still knows.
Conclusion tomorrow. And yes, I really mean tomorrow, before the show.