Jigsaw is a truly spectacular piece of Interactive Fiction that defies imagination. You play a character known only as White, following this person called Black around. Black has this idea that the world would be a better place if someone took a time machine and changed a lot of things. You, of course, must stop Black, since our history is the best of all possible ones. Other than that, it's a very impressive game. See http://www.pond.com/~russotto/zpletx/jigsaw.html