Talk about spot-on casting. Gordon-Levitt was excellent in Johnson's first film, and is no longer simply the 3rd Rock kid-turned-indie darling. He's got G.I. Joe, Elektra Luxx, and Inception under his belt, and really, is an obvious and apt fit.
THR got the plot a bit mixed up, describing it as the story of operatives sent to the present to assassinate criminals. According to Johnson (who described the film for Cinematical Movie Club in a post that goes live tonight), it's about a group of killers who kill targets as they're sent back into time. So, unlucky dude zips back from the future, is killed pronto, and in the future, there's no body or proof to the deed. Sounds awesome.