You could be forgiven for thinking that a film about how two people spend their last three weeks before an asteroid collides with Earth would be a sci-fi drama. But despite the high-concept, Seeking a Friend for the End of the World is nothing more or less than a delightful romcom. Rammed with wit and warmth, it delivers heartily on the rom (the apocalypse acting as a metaphysical Cilla Black) and com (the apocalypse is a rich source of fatalistic humour) fronts.

This is not a sober Melancholia-style cataclysm, it’s more like ‘Naked Gun doesn’t do apocalypses but if it did….’ All is good-natured chaos, from the end-of-days countdown on the nightly news to the public posters offering the opportunity to fuck a virgin. Plodding round this circus is Dodge (Steve Carell) who’s in no mood to seize the moment as his wife has just literally run off. While his middle-aged friends throw Eyes Wide Shut-for-embarrassing-dad-style parties where they take acid and tear up the rulebook on fidelity, he stoically continues to go into work. The only revelation that takes root as his certain death grows closer is that he should seek out his high-school sweetheart, the lost love of his life.

So far, so downbeat, but then Keira Knightley appears in full Clementine from Eternal Sunshine…, Manic Pixie Dream Girl mode. Her character Penny is a record-loving, impulsive, hypersomniac and the pair end up involved in a buddy-movie road trip as they try to get to the people that matter the most before it’s too late. It’s no surprise that Carell, whose eyes are brown pools of kindness mingled with weariness, holds his own as a downtrodden wit with a big heart (See Little Miss Sunshine and last year’s Crazy Stupid Love for previous form). The revelation is Knightley who, although not quite lost in her character, has an infectious energy that sets the pace. Both she and the film possess the good-natured gait of a puppy that’s never been kicked and they race towards the finishing line with giddy abandon.

While the film is about the blossoming relationship between this oddball duo, Seeking a Friend for the End of the World gives generous time to the supporting cast and their subplots. Whether it’s the emotional truck driver for whom three weeks is still an eternity to wait to bid goodbye to the cruel world or Patton Oswalt’s opportunistic sleaze-ball bowled over with excitement at abounding sexual openness, these colourful characters provide regular blasts of entertainment. A climate of hysteria rises and falls and our leads’ ability to empathise with each other in these extreme circumstances makes us root for them.

Some developments feel unlikely but the logic of ‘this is the apocalypse’ sucks up and owns all bizarre swivels. Warmth pulses from every frame. Penny’s record love manifests in classic after classic. Wardrobe and colour palette are all gentle pastels and solid primaries. Liberally sprinkled are cameos so delightful and unexpected that your face will break out in smiles.

It’s amusing and a little odd that such a tragic concept is given such wholesome treatment (there’s a white picket fence knocking about) but once you see through the disguise of the subject to a solid and heartfelt romcom, all is well. This is a well-written, warm treat of a film about a pair whose unlikeliness is aspirational to anyone who wants to believe that love can grow against all odds.

Contributor Sophie Monks Kaufman can be followed on Twitter @Sopharsogood.