Stuck for a clown at his son's birthday party, an all-American family man and real estate agent decides to do the job himself, dressing up in the clown get-up he finds in one of his properties. No, this isn't an episode of Modern Family - what goes down well with his son and the local kids soon takes a more sinister turn as Dad finds that he can't get the makeup or costume off, beginning a physical and mental transformation from typical family guy to demonic clown murderer. It's like The Santa Clause, but with an evil murder clown instead of Father Christmas.

The intense gore and body horror sit ill at ease with a story only a few steps removed from a Goosebumps episode (The Haunted Mask has a very similar conceit) and it doesn't help that none of the characters react to anything logically or believably. There's certainly humour to be wrought from such a concept, but Clown doesn't do a good job of finding it, playing Dad's transformation far straighter than the metamorphosis of man to clown ever should be. The first half is where Clown is at its hardest to like, frequently misjudging its own tone and handling of the story. Peter Stormare pops up to lighten the load – like the equally silly Bad Milo, it's not really worthy of his talents, but his presence is welcome, nevertheless. It's when the titular clown takes over that the film is at its most enjoyable, doing away with most of the stupid attempts at humour and letting the impressive gore and makeup effects do their work. That the clown's victims are largely, naturally, children gives an unsettling element that works far better than its trying to be funny or zany. There's still black humour to be had (particularly in its Killer Klowns-esque ice cream brains and bloodshed) but it's much better left in the background than when it's allowed to take precedence.

But then, producer Eli Roth (who also cameos unrecognisably as clown Frowny) would know all about that, having shot most of his own movies in the foot with a strain of dumb fratboy humour that he couldn't even lose in the verdant depths of The Green Inferno. Clown is less immature than that, but director Jon Watts can't stick a set of likeable characters either, nor anything resembling realistic human behaviour. What it does have, at least, is a great killer clown – so effective that the movie's poster was banned in Italy because of it. The clown looks the part in all stages of his transformation, from home-made greasepaint beginning to fully-fledged giant demon end. It's a great horror villain, and one which will have Coulrophobics shivering before the screen. Have we ever had a truly great killer clown movie? Even Stephen King's IT, the classic of the subgenre, is a deeply flawed piece, these days coasting on nostalgia and affectionate memories rather than genuine brilliance. Too many horror films are content to take that imagery and do nothing with it, letting the audience's own fears do all the hard work. At this stage, killer clowns are the horror movie equivalent of dwarves in a Movie Movie or Farrelly spoof – just as little folk aren't a punchline themselves, nor should a scary bit of clown makeup be the only thing your horror film has up its sleeve.

What could have been one of the few great killer clown movies winds up as yet another disappointment, being too uneven in tone and pace to be considered a success. It livens up for the finale and several points in-between, but has spent too much time by then floundering in useless comedy and a terrible attempt at clown mythology. The gore, impressive body horror and unusual story make it impossible to completely dismiss, but Clown could have been so much more.