Ianto Jones knows about the apocalypse. He's read about in books. He's seen it on TV. And watched just about every movie ever made about it.
And he's made some rules for himself. You know, to follow during the apocalypse. He repeats them silently in his head when he's in really tight situations. Or when he's nervous. Or bored. Or scared shitless (which happens more than he'll ever admit).
Ianto Jones knows a lot about what to do in, and what not to do in, an apocalypse. Who survives, who dies, who lasts almost to the end, who gets to drive off with the girl. He's keeping track, keeping it all straight in his head, keeping a fucking to do list for the apocalypse.
Ianto Jones knows there'll be an apocalypse. Although he's not sure just when it'll be coming, he certainly knows that it does. Because Ianto Jones has already seen the apocalypse. Not too long ago. From the doorway of a deceptively small blue police box.
