Five minutes into The Cabin In The Woods I was feeling the dread of the unspeakably naff.

With the title presented in blood-red capitals and the introduction of bland, nubile American teens who were clearly lined up as fresh meat for terrifying slayers, this had the makings of just another slasher movie. The next 90 minutes stretched before me like a migraine.

And yet, I couldn't shake another feeling: hope. This came with the knowledge that the film's co-writer was none other than the great Joss Whedon – creator of Buffy The Vampire Slayer, Firefly and Dollhouse, and a man who customarily subverts the genres he's working in.

And here on screen were old hands Bradley Whitford and Richard Jenkins, class actors who one wouldn't expect to see in slice-and-dice horror. So I held my breath. And I was duly rewarded. This film doesn't just subvert its genre, it blows it apart, with wit, ingenuity and a cheery gusto that eventually leaves the viewer in loose-jawed astonishment.

The build-up to the action establishes a certain enigma. The teenage group heading for a weekend of sex, drugs, torture and dismemberment are the stock types: the sex-hungry flirt with "first victim" plastered across her forehead, the sweet girl who will survive the longest, the hunk, the sensitive guy, the geek.

But their scenes are intercut with those featuring Whitford and Jenkins, as scientist-bureaucrats working in a high-tech complex, who seem to have the kids' fate in their hands. How, and to what purpose, are the questions that keep us intrigued.

There are other early signs that this isn't going to be the usual hillbilly nightmare – such as the eccentric petrol station attendant whose fire-and-brimstone pronouncements become absurd when he is put on speaker-phone.

What follows is grisly enough to satisfy those who like their exploitation horror served raw, light enough to enable the rest of us to relax and enjoy the dry humour and wild, wacky imaginative leaps.

It's especially pleasing to see Whitford, so wonderfully funny as Josh Lyman in The West Wing, exercise his superb quip delivery on the big screen. He and Jenkins preside over the best jokes and surprises, from the office sweepstake on which monsters will ultimately kill the kids to the revelations of the end of the world as we know it.

It's 11 years since Audrey Tautou made her name as the wide-eyed and kind-hearted Amalie. The actress has long since grown up, the naïf becoming a mature woman with, in fact, a prematurely pinched-mouthed severity. But the filmmaking Foenkinos brothers of Delicacy seem desperate to incorporate the old and new Tautou into the same character. It's a mistake that sums up a very conflicted film.

Tautou plays Nathalie, whose perfect life is thrown into disarray when her husband (and, by all accounts, soul mate) dies in an accident. The story follows her extended mourning, in which she launches herself into her work and repels romantic advances, until the most unlikely of men creeps under her defences. The film, based on David Foenkinos's novel, wants to have its cake and eat it: to be a serious tale about grief and loneliness, and a whimsical, slightly oddball comedy. The result is tonally all over the place, and nowhere near as charming as it thinks it is.