The indie comedy “Safety Not Guaranteed” has a terrific premise that it doesn’t entirely know how to execute; whereas it might have taken off into a whole new dimension (literally), it instead is content to serve up the same old, same old.

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 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,” which was one of the big audience hits at this year’s Sundance Film Festival, 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 movie was written by Derek Connolly and directed by Colin Trevorrow, both making their feature debuts.)

The cast makes it watchable even as the story devolves into nonsense.

Johnson (from TV’s “New Girl”), is a terrifically endearing cad, effortless with a one-liner — he brings to mind a young Vince Vaughn.

Plaza (from “Parks and Recreation”) 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 (who, with his brother Jay, is best known for writing and directing “Jeff, Who Lives at Home” and “Cyrus”) 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.