Contrary to popular belief, Captain Jack Harkness loves happily married couples, even when he can't talk his way into their beds. They're just so cute, with the snuggling and the private smiles and loving looks. So happy that they radiate contentment, so happy he feels only vaguely uneasy about trying to leech some of that happiness for himself. Anyway, most of them love to share.
This one is cuter than most, in a remarkably understated way. He can understand that; they had survived a Dalek invasion that had been defeated by the Doctor (and Jack, of course, working undercover on the other side of the planet). Dark-haired, both of them, he conventionally handsome and she with the face of a pleased pixie. Beautiful, and beautifully happy, both of them.
She looks familiar. He cannot say where from.
Jack puts on his most handsome smile and approaches them, his hand out. "Captain Jack Harkness," he says.
The man shakes his hand, firm and hearty but not overly hard. "David Campbell," he says.
Jack turns to her, and she presents her hand to be kissed with a whimsical delicate flair, and suddenly he knows her.
"Susan Campbell."
"Susan," he repeats, and smiles.