Ironically for a film whose publicity campaign has centred around the glinting metallic grimace of Jared Leto's psychopathic Joker, Suicide Squad is entirely without bite. Writer/director David Ayer is a filmmaker known for his ability to get his teeth into the brutalities of conflict, both on the street (Training Day, End of Watch) and in the field (Fury), but here, for whatever reason – studio meddling, ratings concerns – there's the definite sense that he's been creatively muzzled.

There's the bones of a deliciously dark, decidedly adult film in this story of a group of incarcerated supervillains recruited to undertake dangerous missions - their first one against the black magic of a world-conquering ancient witch. Despite the promising moral mire of the narrative, however, the film plays it boringly safe and, for a team of dangerous villains, this bunch exhibit behaviour so conventional it borders on the twee.

All Will Smith's crack assassin Deadshot really wants is to be a good dad to his daughter. Diablo (Jay Hernandez) spends the entire film moping about the fact that his fiery temper turned his loved ones to ash. And for all his forced mania, Leto's camp Joker is a grating, one-note pantomime villain displaying none of the psychological complexity that the late Heath Ledger brought so memorably to the role in Christopher Nolan's Dark Knight.

The women are, of course, treated far worse. More attention is paid to Harley Quinn's (Margot Robbie) ass-grazing hot-pants than her character; a formerly professional psychotherapist driven insane - indicated by smudged make-up, a broad New York sing song voice and a propensity for violence - by her inexplicable love for The Joker but who, we discover in a hallucination sequence, wants only to be married with kids and rollers in her hair. Similarly, the one time Karen Fukuhara's samurai master Katana takes centre stage in a scene, she is mourning the soul of her dead husband which is trapped in her sword. And Cara Delevingne's writhing Enchantress is less fiery demon and more damp squib.

The cast do what they can, but are stymied by a barrage of unconvincing effects and a lacklustre screenplay, which sees these supposed 'worst of the worst' causing no more than the normal amount of mayhem while spouting schmaltzy cliche-ridden dialogue. ('We're bad guys. It's what we do,' grins Quinn after – shock, horror – smashing a window.) While it might be good to be bad, Suicide Squad is just plain awful.

Wide release from Fri 5 Aug