Aliens have invaded the world (possibly by accident) and are doing an exceptional job of conquest due to their unique nature. Tom Cruise, who is nowhere close to a battle ready soldier, is sent to the front and by chance gains the ability to reset the day... by dying. This actually takes away some of the concern the audience may have for the characters and indeed it is played out for some pretty funny laughs. The resets are also a clever way to reuse CGI with just a few changes, and one I didn't mind at all.
His multiple deaths make the movie better than it should be.
Apart from the time shifting part though, it's a pretty standard fare for an "alien invasion" theme where they happen to have one (or very few) lynchpin(s) that, once overcome by the heroes, suddenly means the aliens are screwed. That's not to say it's a bad movie. It's pretty entertaining, violent, and pretty cool to see Cruise's character improve and get ever more tired with each iteration of the day. I give it three funny deaths out of five, and while I'm not rushing out the door to buy its DVD, I wouldn't mind watching it again.