This film screened at the South by Southwest Film Festival.

Safety Not Guaranteed isn’t what you’d expect it to be. It’s a bittersweet comedy that flirts with time travel, but it’s not straight science-fiction or rom-com. It resolutely refuses to tie up a couple of its plot lines, yet the story is still satisfying and full. Most rewardingly, it’s a dramatic comedy built on relationships that feel earned, nuanced, occasionally uncomfortable, and completely relatable. Director Colin Trevorrow, in his first feature, mines a series of relationships for small-scale humor and poignancy, and the script from Derek Connolly (also his first feature) has some wonderful moments that reflect the awkwardness of young adulthood and the way we all eventually have to reckon with the choices that we make. The film is light and often breezy, but it’s anything but insubstantial.

Darius Britt (Aubrey Plaza) is a twentysomething adrift in the professional world. She’s an intern at a Seattle magazine, but she’s doing it more to prolong making any actual decisions about her life than to get ahead. She runs errands and fetches supplies, and though she hates her boss, she doesn’t really act as if she’s suffering. Minor injustices roll by as part of the scenery. It’s not that she’s not interested in telling good stories — her motivation to do more is what sets the larger plot in motion — it’s that she seems equally comfortable coasting until those moments come along. Effort is spared until the last possible moment, and at that point it’s fraught with complicated and possibly imagined subtext. In other words, she’s in her 20s.

At a story meeting one day, senior writer Jeff (Jake Johnson) pitches an idea to follow up on a classified ad he found:

“WANTED: Someone to go back in time with me. This is not a joke. You get paid after we get back. Must bring your own weapons. Safety not guaranteed. I have only done this once before.”

Darius volunteers to help him research the ad’s author, so Jeff, Darius, and an intern named Arnau (Karan Soni) head up the coast to the small town where the author lists his P.O. Box. It’s a workable premise for a road movie, and a good way to get the characters out of their comfort zones, but Trevorrow is more interested in the moral underpinnings of their journey than in its outcome. Darius eventually tracks down the ad’s author, Kenneth Calloway (Mark Duplass), to discover that he’s unsurprisingly weird, and awkward, and seemingly unmoored from the world around him. She plays along with him in an attempt to get his story, but — naturally — things get more complicated.

What keeps the film from becoming a rote coming-of-young-age story is the way the script gives just as much weight to Jeff’s ulterior reason for taking the trip: He wants to hook up with a former flame he’s recently reconnected with on Facebook. Jeff’s a swaggering ass who loves his car and his clothes, and the woman turns out to be grounded and honest, and it’s their fledgling rekindled romance that forces him to grow up and that really gives the film a sense of reality. It’s a side plot that doesn’t have much to do with the main one — it’s only Jeff’s gradual edging into maturity that connects them, and then only tangentially — yet its emotional balance to Darius and Kenneth’s growing relationship turns the film from a by-the-numbers story into an observation about the cost of getting older and the lengths people will go to just to avoid heartbreak.

Kenneth wants Darius to help him go back in time so he can fix something in his past. “This mission is about regret,” he tells her, which is the saddest and truest thing anyone ever said in a story about time travel. Who hasn’t thought of going back to change something, to say something, to help someone? Kenneth’s goal isn’t scientific discovery or financial gain; it’s freedom from the same burdens we all carry. Duplass is great in the role, too. He’s quiet and uncomfortable, but never foolish. He plays Kenneth as a real man, not a cartoon or a psycho. Plaza makes a great counterpoint for him, too. She’s never really been given a role that’s asked her to dig too deeply below the sardonic surface she’s perfected on “Parks and Recreation” and other outlets, but here, she successfully comes across as the right mix of insecure and honest. Duplass is eight years her senior, but they have a pleasant, warm chemistry together, and their connection is what gives the film its resonance.

There’s an axiom that the hardest acts to realistically portray on film are sex and prayer. Surely, though, falling in love is a close third. Countless films take for granted that the male and female leads need to be together for plot reasons, and thus forget to bring them together for emotional ones. We accept this most of the time, and we usually settle for relationships that bear some resemblance to real ones even as they make it clear they’re just part of the larger story. It’s rare for a film to actually capture the moment of falling in love, and rarer still for that moment to be the entire point of the film around it. Safety Not Guaranteed starts out with a modest premise but eventually grows into something weirder and stronger and more self-aware, and its actors create rounded characters who experience real joy and loss. The final product is something unexpected yet familiar, a new version of a welcoming story.

Daniel Carlson is the managing editor of Pajiba and a member of the Houston Film Critics Society and the Online Film Critics Society. He tweets more often than he should, and he blogs at Slowly Going Bald.