Sacha Baron Cohen changed the shape and texture of film comedy with Borat and Brüno and then settled for being merely hilarious in The Dictator. His new film Grimsby – which until recently was titled The Brothers Grimsby – doesn’t even manage that. It shows signs of poor comic judgement, hurried patchwork editing and the sort of uncertainty of tone that is only to be expected when you hire an action filmmaker (Louis Leterrier of Transporter and Incredible Hulk fame) to do a comedy director’s job. The vocabulary of the comedy world may suggest violence (comedians “kill” or “slay” their audience) but the two genres require different skillsets. Leterrier doesn’t know how to frame and time and edit gags, or how to let them land. He’s so wrong for the task that it’s almost funny. Almost.

Grimsby bolts together several familiar elements. It takes two chalk-and-cheese siblings – the feckless layabout Nobby (Baron Cohen) and his long-lost secret-agent brother Sebastian (Mark Strong) – and launches them into a spy caper with shades of Bond, Our Man Flint, Spy and MacGruber. The emphasis is on the gross-out. In one scene, Nobby is called upon to suck poison from an intimate part of his brother’s anatomy. The movie’s big set-piece, which I won’t describe here, seems conceived solely to outdo the moment in The League of Gentlemen Apocalypse when a giraffe’s prostate gland is over-stimulated with spectacular and sticky results.

Baron Cohen and his co-writers will stop at nothing to soak their characters in bodily fluids or have them admit foreign bodies into their orifices. It would be wrong to say any of this is funny exactly – the contrivances are too obvious to allow these scenes to ambush us fully. When a family of holidaymakers on safari in Ace Ventura: When Nature Calls saw a rhinoceros apparently giving birth to a naked Jim Carrey (it’s a long story: just watch the clip), the moment was all the more sublime for having been arrived at without hints or fanfare. Before we knew it, we were in the midst of the uproarious spectacle.

Grimsby isn’t like that. At all times its workings-out are visible. Only one moment, in which a man and a woman talk at cross purposes (she’s come to seduce him, he thinks she’s there to unblock the toilet), has the necessary component of comic delirium. It wouldn’t be out of place in a Carry On film.

No one was sure whether Baron Cohen would be able to survive as a film comic after ditching the semi-documentary thrills of Borat and Brüno. But The Dictator landed some solid punches to the face of western complacency, especially with its chastening climactic monologue reminding us how many of our freedoms have been surrendered in the name of security and democracy. Grimsby tries for a similar moment of sobriety under cover of silliness, when Nobby gives a speech in front of a crowd of football fans in defence of the working-class – or “scum”, as one of the villains has labelled them. “It’s scum who built the hospitals they’re closing down!” he rages.

This time, though, the serious points don’t stick. Baron Cohen is mistaken if he thinks this eleventh-hour sweetener will make up for the previous hour or so of class libel. It’s clear that the filmmakers have seen Shameless – they’re indebted to its vision of a ramshackle but lovable dole-queue dad with a brood of grubby, rascally children – but they have taken all of its iconography and none of its wit or sparkle. This is the working class not of Shameless but of Martin Amis’s Lionel Asbo and Channel 4’s Benefits Street and Harry Hill’s hateful comedy song “I Wanna Baby”.

One way for Baron Cohen to defend the characters derided by the film’s villains as “scum” would have been to have given them something interesting or amusing to do, rather than rendering them as a faceless rabble. As it stands, the sight of Ricky Tomlinson and Johnny Vegas being dragged on to deliver the odd perfunctory line while the star gets the gags is awfully depressing. Still, we’ve all got to work. Perhaps we’ll see their big scenes among the DVD extras.

What makes Grimsby an especially steep falling-off after Baron Cohen’s last three movies is the sense that no one really cared whether it came off or not; the whole enterprise has a “will-this-do?” quality. The stop-start plotting is detrimental to any comic momentum, the guest stars (Rebel Wilson, Gabourey Sidibe) are thrown away, the effects work looks nothing short of rinky-dink. And there is actual amateurishness here, which is the last thing anyone associates with this notoriously meticulous comic. The opening scene is supposedly set in Grimsby itself but no one involved in the film seems to have noticed that Nobby clearly boards a bus to Grays or that several signs are marked Tilbury (both are towns in Essex).

Chances are this was spotted in the editing but the filmmakers must have believed viewers would be too stupid to care. It’s one thing to look down on your characters. Despise your audience, though, and you’re really having a laugh.

“Grimsby” is on release.