In September of 1997, little known publication Backwoods Home Magazine ran a rather mysterious advert in its personals section. Written (as a joke, disappointingly) by a senior editor, the ad read “Wanted: Somebody to go back in time with me. This is not a joke. You’ll get paid after we get back. Must bring your own weapons. Safety not guaranteed. I have only done this once before.” Fifteen years on, a film inspired by this odd little event has arrived. Suitably quirky but never teeth-grindingly twee, Colin Trevorrow’s film is an funny, engaging watch. But as the credits roll you may well find yourself left wanting more.

Darius (Aubrey Plaza) is a disillusioned magazine intern who spends her days stocking up the office loo roll, being patronised by her shrewish boss and going home to eat takeaway with her dad (a brief cameo by Jeff Garlin). Then one day at work, sleazy journo Jeff (Jake Johnson) pitches a story – he’s seen a personals ad seeking a time travel companion and wants to investigate. Jeff chooses to bring Darius and her co-intern Arnau (Karan Soni) along for assistance and before long they’ve tracked down the man behind the advert. Enter Mark Duplass as loveable oddball Kenneth, a shelf-stacker who spends his spare hours preparing for his next time jump and seeking a suitable companion to accompany him. Darius is quickly volunteered by Jeff to infiltrate Kenneth’s world and get the scoop. Inevitably, though, she soon begins to enjoy Kenneth’s company.

In much in the same vein as last year’s Another Earth, the relatively small world of Safety Not Guaranteed is bound up in and made larger by the universal parallels it draws upon. In both cases, the sci-fi elements stand in as metaphors for emotional crises. Here, though, the wish isn’t only Another Earth‘s “If only we could go back and change the things we did wrong”, it is the more basic and thus the more poignant “If only we could go back“.

One of the most involving scenes is where Kenneth details his reasons for investigating time travel, describing – with a pained wistfulness – the way in which a particular song can take you back to a certain time in your life. Darius, agreeing, finds herself drawn into Kenneth’s sad, strange world – a world of regret and sadness but also, crucially, of hope. What’s more, it’s a hope that is infectious. Even the obnoxious Jeff finds himself rekindling the flame of a lost teenage relationship. As becomes apparent, Jeff is attempting to do some time-travelling of his own, consumed with the desire to return to the good old days when he was young and carefree – indicated by his constant pressure on the young Arnau not to waste his youth. Never is any of this hammered over our heads but, oddly, whilst the film’s subtlety has its own quiet appeal, it also pulls us back from ever really investing in this odd little story.

Part of the problem stems from the central performance. Plaza’s schtick, which she has spent the past few years perfecting, is the bored, half-lidded look; the kind of disaffected scorn that lends itself well to modern naturalistic comedy. And while that works in certain portions of the film it also means that moments requiring emotional intensity from her fall a little flat. There’s just something about Plaza’s face that renders her incapable of emoting to the level required. But it’s also telling of a film which, for the most part, underplays things. In truly modern style – as popularised by the likes of Duplass and his brother Jay – the action shuffles forward in the smallest ways, producing the biggest audience reactions through the comic moments.

Ultimately, the main problem with Safety Not Guaranteed is that in its genre-straddling, something feels lost. It never quite fully commits to the story at its core and – with an end sequence that feels rushed and, dare I say it, actually a little amateur in its execution – Trevorrow’s film seems to have made the decision to veer away from any definitive conclusions. I don’t like to bandy around the term “cop out” too freely, and it feels unfair to reduce what is a sensitive, nuanced, thoughtful film to such a phrase. But it’s frustrating to encounter a project with so much potential that never quite follows through on its initial promises. Safety Not Guaranteed hooks you, but never quite reels you in. And that’s a big old shame.