YouTube hasn’t been the best breeding ground for cinematic talent, as 2010’s Fred: The Movie, which grew out of a hit on the video-sharing platform, will attest. This much-hyped inner-city drama – the debut of Rapman, AKA Andrew Onwubolu, whose web series, Shiro’s Story, reached 10m hits – is an assured and capably performed morality play.

Tackling a vicious outbreak of gang violence on the border separating Peckham and Lewisham in London, Onwubolu ports across many of the elements that made his online endeavours such a success – chiefly a sure feel for the south-east London streets and his own rapped on-camera narration.

This last element is where Blue Story is most innovative, elevating a stock melodrama – centred on childhood pals Tim and Marco (Stephen Odubola and Micheal Ward), set in fatal conflict by their older brothers’ affiliations – into exhilaratingly musical territory. (Turn the dial, and we’re not too far from Greek tragedy, or Baz Luhrmann’s Romeo + Juliet.) The device is at least as dynamic as the scuffling, which erupts out of nothingy Tuesday afternoons, or indeed Onwubolu’s sometimes lurching narrative. A “three years later” card placed at the halfway mark allows for closer scrutiny of the consequences of street violence, whereupon Odubola – pick of the film’s promising new faces – reinvents likable Tim as a brooding avenger.

If there’s a limitation, it may be in a modesty of means and spirit you might not expect from a film-maker newly signed to Jay-Z’s Roc Nation label – it’s a headscrambler to see a Paramount Pictures production with scenes that take place outside a Greggs.

But this is true to the way Onwubolu avoids the usual flash and posturing in favour of a careful, rooted storytelling, finding subtly different perspectives on gang life, and offering his characters as many ways out as there are ways in.

• Blue Story opens in UK cinemas on 22 November.