I can pinpoint the precise moment that the Rose Theatre’s new musical Junkyard comes to life. Halfway through the first act, idealistic teacher Rick (Calum Callaghan) has finally convinced a scraggly, reluctant gang of young Bristol teenagers, led by Fiz (the spunky, ebullient Erin Doherty), to help him build a local playground. As the youths overcome their cynicism and suspicion, the run-down junkyard suddenly transforms into a collection of instruments, and the kids start banging and clanking joyfully while they sing, the musical theatre form catching up at last to transform their tuneless lives.

Junkyard, with a sly, insistent, and often hyper-catchy soft rock score by Stephen Warbeck, and whip-smart book and lyrics by Harry Potter and the Cursed Child’s Jack Thorne, shakes off all sense of the ordinary. Authors and characters alike do battle with the borders of appropriate musical theatre behavior: some songs last only two lines and actors glare at the musicians for failing to come in at the proper cue or for interrupting a perfectly good spoken scene. Akintayo Akinbode on bass leads the rollicking band with Nadine Lee rocking out on a variety of onstage and offstage percussion instruments, and Dario Rossetti-Bonell on guitar.

Thorne provides weird, thought-provoking lyrics that range from the rhythms of daily speech to the quietly poetic: “Courage sits waiting, waiting in a cave / It’s got no friends.” Rhyme is usually sacrificed on the altar of rhetorical authenticity and, for this show, that decision works. The story he tells unabashedly entreats the audience to support opportunities for kids to be kids and have the chance to play – the Adventure Playground movement depicted here is one such example – but the Bristol world shimmers with such truthfulness and nuance that the ultimate call to arms doesn’t feel didactic.

Standouts from a uniformly tremendous cast directed with a whimsical, energetic touch by Jeremy Herrin include Enyi Okoronkwo as Talc, Fiz’s gentle and devoted friend, and Josef Davies as Ginger, a hothead who lashes out at his classmates for singing backup during his solo number. Ginger has bullied Talc in the past – he’s the source of that body odor-related nickname (short for talcum powder) – and their scene of reconciliation movingly punches through the performances of adult masculinity that these young adolescents feel obliged to put on.

Rounding out the motley playground crew are Tilly (Seyi Omooba, charming), Higgy (Jack Riddiford, convincingly both macho and tender), and Loppy (Ciaran Alexander Stewart, winningly geeky). Lisa Palfrey as Fiz’s errant Mum and Kevin McMonagle as the overworked headmaster delightfully humanise the older generation, while Scarlett Brookes brings a witty wistfulness to Fiz’s pregnant sister Debbie.

The cast – young and old alike – scrambles over the precarious but fun-looking set by Chiara Stephenson (a piece of it did collapse unintentionally on press night adding to the visceral sense of potential playground peril). Jack Knowles’ lighting design experiments effectively with illuminated darkness – candles and flashlights, and fires abound. Polly Bennett’s down-to-earth choreography supplements Herrin’s stellar staging.

Like the playground the kids build and the lives they lead, Junkyard is jagged and imperfect, never entirely on firm footing. On the purely practical level, diction goes in and out – I wish I could have caught every word. Until the playground-building gets started, Warbeck’s melodies tend to consciously literalise the kids’ boredom with Bristol life which makes for some monotonous tunes early on.

The show struggles to clarify its final judgment on Rick’s role in the community; while Rick’s own self-doubts about his work make up a good deal of the second act (“You want to save us like rhinos or arctic penguins,” Fiz tells him, accusingly), just what (if anything) Thorne wants us to think never quite comes through. It’s also strange that a story that is otherwise unflinching should seem to tiptoe around the issue of race, especially since the show is set in 1979, a moment of high racial tensions in Bristol.

But those rough edges tend to feel in keeping with the show’s treatment of the form: writing a musical’s a tricky business, like building a playground or healing a community.

Junkyard might lack the polish that would make a West End transfer lucrative, but it certainly deserves to fill the house for the rest of its Kingston run. For an untidily eloquent night out, there’s nothing quite like Junkyard.

Junkyard is playing at the Rose Theatre, Kingston until 30 April. For more information and tickets, see rosetheatrekingston.org.