Bruce Willis already starred in one of the best time-travel movies ever made—Terry Gilliam's 12 Monkeys. But in spite of the specter of Monkeys, Looper is a spectacular genre thriller, perhaps because it employs the exact opposite theory of time travel. Whereas in Monkeys, Willis was stuck, immutable, a slave to destiny, in Looper, the future is marvelously fluid.
Joseph Gordon-Levitt stars as Joe, a man employed by criminals from the future to do their janitorial work. He stands in a field, holding a beautiful, junky gun—Looper's design is exquisite—until a bound, gagged prisoner from the future appears. Joe immediately blows the prisoner away and destroys the body. He's been doing this for years. (In the future, it's impossible to get rid of a corpse. It's also illegal to travel in time, but when time travel is criminalized, only criminals will time travel.) One day, the prisoner is Joe as an old man (Willis). All hell promptly breaks loose.
To delve further into the plot would strip Looper of its twisty thrills. Surely, some critics will pull the film's logic to pieces, but that's hardly the point. Looper is smart, but it's not indestructible; we're not dealing with Primer here. But the way writer/director Rian Johnson (previously known for Brick and two excellent Breaking Bad episodes) lets the present affect the future—a nauseating torture scene is a new time-travel classic—keeps the tension high.
And Gordon-Levitt's performance as a young Willis is brilliant. With the help of some obtrusive-but-still-impressive facial prostheses, he delivers a pitch-perfect Willis impression, nailing Willis's twisted, self-satisfied smirk and his bemused line deliveries. It's not just mimicry—Gordon-Levitt didn't just photocopy old Moonlighting episodes—it's a bold, original act of creation. And so is the rest of Looper. If you've ever complained about Hollywood's lack of originality, especially as it pertains to genre movies, you owe it to your past self to go see Looper once or, preferably, twice. 
