James Cole (Bruce Willis) is a prisoner in the future. His society lives underground because a man-made virus has killed off everyone above ground. He's recruited by the scientists to get samples on the surface--and we see a city overrun by nature and inhabited by bears and lions. Then he's asked to volunteer for another, more complicated mission-- to go back in time and get a sample of the virus before it mutants. Once they get this they can work on a cure and perhaps reclaim the surface world.
Yet things go awry and Cole ends up in a mental institution. It's here he meets Doctor Kathryn Railly (Madeleine Stowe), who later helps him, and inmate Jeffrey Goines (Brad Pitt). Cole realizes they had sent him back in time a few years too early--and so they yank him back. On the second trip he has just enough time to stop the "Army of the Twelve Monkey", who the scientists think is responsible for unleashing the deadly plague. And Goines is the one in charge of this organization. Cole feels responsible because he planted the idea in Goine's head all those years ago, which means he's responsible for the near extinction of the human race.
This is one of the better time-travel movies out there and though fifteen years old hold up over time, no pun intended.