This is perhaps the best single issue of a comic book I've ever read. Kurt Busiek crafts a simple but thoughtful little love story, and Brent Anderson's art is spot-on.
An average citizen of Astro City named Michael is having trouble sleeping and dreams of a woman he thought he knew somewhere named Miranda. It turns out that due to change in the time line, Miranda did actually once exist and she and Michael were lovers. But, when the time line was restored (after a fight between costumed heroes and villains), Miranda ceased to exist, but Michael's memories of her lasted. Michael, vested by the Hangman (a being that can erase memories and much more) is offered a chance to forget her, but doesn't. Michael chooses to remember her, and finds his peace in his memories of her.
It's a touching little love story and time travel story done right, because it's a simple story, with characters we are made to care about. It's also a story done well in only 16 pages.