For about forty minutes, 21 Bridges is a compelling film.

Those forty minutes – its first, minus a brief prologue establishing the policeman bona-fides of protagonist Dre (Chadwick Boseman) – are tightly-executed, detail-oriented cops-vs-robbers thriller. A pair of bad guys (Stephan James and Taylor Kitsch) steal some cocaine and kill a lot of cops in the process; the island of Manhattan’s bridges (all 21 of them!) are shut down until sunrise, and the chase is on. The criminals are compelling, the narrative sharply paced: it’s good, pulpy stuff.

Until, somewhere around the halfway point, director Brian Kirk (confusingly, not the Russo brothers) gives up on this premise to slide into a paint-by-numbers corrupt cops story without an iota of ambiguity. Dre’s good, the other cops are bad: let ‘em fight. Snore. It doesn’t help that the film sets up Boseman’s character as morally grey – he’s killed ten perps over as many years – but sanctifies him as the One Good Cop. That’s especially egregious in the context of the climax, which after an hour-and-a-half of underlining the importance of seeking justice without firing bullets, erupts into an implausible firefight where the better gunman wins the day.

At least you’ve got those forty minutes.