Joe (Joseph Gordon-Levitt) is a specific type of hitman in 2044. People from thirty years in the future are sent back in time, to an exact location, and when they appear he kills them. However, things don't go as planned when his future self (Bruce Willis) is transported. What is supposed to happen is that this "closes the loop" and his services will no longer be needed, though he knows he only has the next thirty years to live before he's killed (by his younger self). But his older self escapes and so he has to find him--if he doesn't, he'll be killed far earlier than he's supposed to.
The first half of the movie is interesting--but the second half turns into a completely different story, involving a single mother and her strange kid, who is able to move objects with is mind. This kid grows up to be a criminal in the future who ends up killing all the Loopers. So, does Bruce Willis kill him or not? Or does he do something that will make the kid become a different person? For me, everything is too unlikely and as a time travel movie I really didn't care about the main plot points. Looper is a pooper.