"You're going to die."
Jack looks up at him and away from the lock he's been fiddling with, eyebrows raised in slight confusion. Maybe right now isn't the best moment to bring the topic up, while he is leaning against a wall and watching Jack trying to pry the lock to their cell open with a hairpin that he has on his person for some inexplicable reason, but then again, is there ever a good time for this sort of talk?
"Yeah," Jack replies slowly, "I do tend to keep doing that. I'm trying to be more careful, though, give me some credit."
"That's not what I mean." Stalling, because now he started it, the Doctor smooths down his suit and tugs at his shirt sleeves, like anyone is going to judge his appearance now in this dimly lit stone prison. "You're going to really, actually die. Permanently."
Realisation dawns on the Captain's face and a quiet "oh" slips past his lips as the movement of his hands stills completely. He seems to struggle with his words for a moment before he asks: "Can you tell me... I mean..."
"You saved a city," the Doctor cuts him off, answering the question before it's fully formed. "Will save. Timey-wimey-stuff, you know. Well, it was a planet, actually. One hell of a way to go."
"That's... that's nice, I suppose," Jack says hesitantly, like he's still coming to terms with the idea that there is an end for him. The way he toys with the hairpin between his fingers makes it seem like the thought maybe even scares him a little, so the Doctor feels compelled to add: "And you're at peace."
The Captain's gaze briefly flickers up to him again as he murmurs, "at peace, huh". Then, a little more distinctly: "How do you know?"
"Oh, I was there," the Time Lord replies. Corrects himself. "Will be." Jack's eyes widen and maybe even lighten up a little at that and he swallows, audible in the silence that fills the air around them. "Till the very end," the Doctor promises.
He doesn't mention that he's not actually aware that it is Jack before him in that moment, and that maybe Jack won't really know that anymore either, because that's not the point, is it? It won't matter, later.
Jack swallows again and then smiles carefully. "That's... good. Alright. That's really good."
The Doctor nods. "Yeah. Just thought you might want to know."
"Yeah."
They fall silent again, with the quiet clinking of the hairpin in the lock as one of the few sounds down here, and then, about a minute later, the lock clicks open and Jack pockets the hairpin again.
"There we are," he says, quiet but cheerful, and begins to pry the door open with his fingertips curling between the frame and the wood of the door itself (because it's not like cells have doorknobs on the inside). Before he pulls it open completely, though, he looks over his shoulder and says: "Thanks." Giving the Time Lord no time to respond to that, he continues: "Now, run?"
He makes it sound like a question, and the Doctor gladly steps away from his wall and smiles as he confirms with a nod: "Run."