“To me, the film is a sexy comedy.” Writer-director Jim Hosking is musing on the appeal of his debut full-length feature. It’s an interesting point of view but, as descriptions go, it does little to prepare anyone for entering the world of The Greasy Strangler, a film whose leads, Ronnie (Michael St Michaels) and Brayden (Sky Elobar), are quite possibly the grossest and most unfathomably peculiar father-and-son team in movie history. Think Steptoe & Son as reimagined by John Waters and you’re edging towards the right ballpark. But still not that close.

Ronnie resembles Karl Lagerfeld after a couple of years in captivity. Brayden brings to mind a gone-to-seed Napoleon Dynamite. By day, the pair conduct hopeless “disco walking tours” of their nameless, featureless, depopulated American neighbourhood (“Kool from Kool & The Gang used to work in that convenience store,” etc). And by night, Ronnie becomes the Greasy Strangler. Which is to say, he slathers himself in grease and manually dispatches blameless victims of his unexplained bloodlust before repairing to a carwash to hose himself down. So far, so sexy.

Hosking is half right in his description, though. Against all odds, this film is hilarious, although enjoyment will depend on broad minds and strong stomachs. When Ronnie’s not out doing dismal deeds, he bullies his hapless son to distraction, often while naked and flaunting a monstrous prosthetic firehose penis. There are conversations of almost transcendent, psychedelic banality: in one scene, several painful minutes are spent discussing the word “potato”. Brayden falls in love with Janet (Elizabeth De Razzo) and Ronnie cruelly, uproariously, seduces his son’s dream woman. And all the while, there’s grease. Ronnie slathers his breakfast grapefruit in grease. Some nauseatingly greasy sausages are rejected for not being greasy enough. A massive, greasy saveloy appears in the middle of a delicate-looking Italian meal. Did we mention that this film was greasy?

We’re like naughty schoolboys drawing obscene pictures Jim Hosking

It’s odd to discover that Hosking and co-writer Toby Harvard are two well-spoken English chaps with a background in ads and the odd horror short, including the Warp-produced Privado for Channel 4. From where did this carnival of grotesquerie spring? “We were just trying to make each other laugh,” says Harvard. “We’re like naughty schoolboys drawing obscene pictures. Hopefully, the imagery will stay with people.” There’s every chance of that; with moments like Ronnie prodding a suppurating hole where the nose of one of his victims used to be, viewers may regard their memory as a mixed blessing.

Jim Hosking. Photograph: Maarten de Boer

So how did this monstrosity come into being? In funding terms, we can thank the support, as executive producers, of Elijah Wood and Brit director Ben Wheatley. In a more practical sense, it sounds like a grimly glorious struggle. “The smells at the various locations were exceptionally challenging,” recalls Hosking. “False teeth would fall out. Grease would get in all the cracks. Fake penises would fall off to reveal their real brothers.” The slippery stuff gets a starring role, they say, for the simple reason that it came up when they were riffing on ideas. “It could have been anything,” says Hosking.

Still, you don’t imagine they could have found anything more gross. This queasiness quotient was perhaps last approached in Troma Entertainment trash-slasher flicks or films such as Waters’s notorious Pink Flamingos. But, really, The Greasy Strangler has a sensibility all of its own. If mere gross-out were all it offered, it would get boring. Happily, there’s artfulness here too, which is probably why it was a buzz film at this year’s Sundance. Hosking and Harvard commit entirely to the grubby demi-monde they’ve created and therefore lend it an inexorable internal logic. It’s a world in which no one is normal and nothing is rooted in reality, so anything is possible – a cartoon realised by actual people. So, for all of its depravity, it remains oddly un-disturbing and even – just occasionally, if you’re prepared to buy in – rather charming, particularly in its depiction of Brayden and Ronnie’s warped co-dependant dynamic. In the end, it has its lardy cake and eats it.

“It’s sweet, innocent and childlike in some ways,” says Hosking. “Everyone behaves in an instinctive fashion without thinking about consequences. I wanted it to be a fun, liberating film to watch because that’s how I’d feel about something like this.” He’s right. While you may never feel clean again, this “sexy comedy” has to be seen.

The Greasy Strangler is in cinemas from 7 October and available to own from 10 October