I hesitate to mention that romance and comedy figure heavily in the amusing "Safety Not Guaranteed," lest someone mistake it as one of the crummy rom-coms that have given the form a bad name.

Of course, most rom-coms contain neither rom nor com, and that's be-com the problem. "'Safety Not Guaranteed" resurrects both elements, and wins us over with inventive characters and storytelling that expose just how formulaic other comedies have become.

The movie takes its premise and its title from an Internet curiosity that made the digital rounds a few years back — someone ran a classified newspaper ad seeking a companion for a time-travel experiment. Bring your own weapons, the ad counseled, and be advised: safety not guaranteed.

In "Safety," a Seattle magazine dispatches cynical reporter Jeff (Jake Johnson) and jaded intern Darius (Aubrey Plaza) to find the man who placed the ad — he turns out to be a paranoid loner (Mark Duplass) who is collecting (also stealing) laser components for a time machine of his own design. He's convinced he's being watched.

"Safety Not Guaranteed" is a movie that lives out the ideals of its title. We think we know these characters — cynical reporter, disaffected twentysomething — their rigid movie profiles and character arcs.

But romance takes hold of them, and takes the story in strange directions. Jeff ends up chasing after an old flame, and withdrawn Darius, who's seen so much disappointment in life, ends up intoxicated by the kooky ambition of the man she's pretending to get to know. Additional character details knit the romantic leads together, and embroider the meaning of the movie's time-travel element.

Around every corner is a scene we don't expect, a joke we haven't heard (plus some inspired visual gags). And yet the movie is comfort food also — it carries the DNA of every Capra movie about a city reporter falling for a small town dreamer (Duplass is a little "pixilated"). And it has the infectious optimism of lost classics like "UFOria."

"Safety" succeeds in the essential ingredient of chemistry, built around the off-kilter personas of Plaza ("Parks and Recreation") and indie fixture Duplass. It's too early to say if Plaza's an actress, since no one's asked her to do anything other than be the pretty slacker with the bored posture and world- weary intonation . But she has an unusual kind of beauty (anglo-Irish, Latina) that the camera loves to explore, and when she does make the hyperspace jump to sincerity and initiative, the results are compelling. I don't know if I liked Duplass as much (I kept picturing someone a little younger and more antic in the role), but he gets the character's jumpy, reclusive detachment.

The movie's ending has confused people, I think perhaps because they take the offbeat casting of Duplass and Plaza as an indication of mumblecore "realism." But the movie is a fable, and it's not about the realities of love. It's about possibilities.