The indie comedy “Safety Not Guaranteed” has a terrific premise that it doesn’t entirely know how to execute. Jeff (Jake Johnson) is a ne-er-do-well magazine writer in Seattle, who suggests to his editor what sounds like a promising story, about a man who has been advertising for people to join him on a time-travel expedition. The thing is, Jeff has little interest in actually writing the story – he pitched to the idea solely because he wanted to go to the town where his ex-girlfriend lives. That leaves his deeply skeptical intern, Darius (Aubrey Plaza), having to make contact with the possibly disturbed, would-be time-traveler Kenneth (Mark Duplass). At least for its first third, “Safety Not Guaranteed” generates an easy charm. But just at the point where the movie seems like it’s going to turn into an off-the-wall science fiction yarn, it settles for a series of mildly ironic, overly chatty scenes in which Darius lets down her guard and starts to fall for Kenneth. It’s another mumblecore-y romance featuring characters who talk less like real people than a screenwriter’s fantasy projection of hipper-than-thou reality. The cast makes it watchable even as the story devolves into nonsense. Johnson is a terrifically endearing cad, effortless with a one-liner. Plaza projects an intelligence and humility that helps to humanize a character that might easily have come off as a hipper-than-thou Juno-ish stock figure. Although his part is the least coherently written, Duplass actually manages to make us care about the lost soul Kenneth. Keep your eye on all three of these gifted young performers, and hope they find their way to more substantial material soon.