The Donmar’s last production, Watch on the Rhine, showed a wartime family grappling with an ethical dilemma in the midst of a bleak historical moment and making a heroic collective stand. This drama about a Welsh family and their principled act of courage in the second world war might be seen as its companion-piece. Set in Cardiff’s multicultural Butetown, otherwise known as Tiger Bay, Diana Nneka Atuona’s play dramatises a racial segregation enforced by white American servicemen, when Black American GIs were prevented from integrating with the white community. Its plot revolves around a fugitive GI, Nate (Samuel Adewunmi), who enters a family home. Gwyneth (Sarah Parish), its matriarch, runs a “mixed” boarding-house and has two mixed-race daughters. She is a far more textured character than Queenie, from Small Island. The drama sets up its dilemma as her household decides whether to help Nate or turn him in. Caribbean cargo-man and boarder Norman (Zephryn Taitte) speaks ironically about America’s part in the war against fascism when the nation perpetuates its own racial fascism. Though this is a valid point there is arguably a false dichotomy set up between American racism and its apparently lesser equivalent in Wales. Nevertheless, this is a fascinating and eminently entertaining production, directed by Tinuke Craig, which unpicks its issues with a lovely lightness of touch. Peter McKintosh’s set design shows the boarding house in and out, and there is song, dance and humour that is all genuinely charming despite the horror in this story. The script is deft, never preaching, but some elements are one step removed from a final polish: the winning humour jars in the later, darker scenes when the stakes are raised; a love story feels tacked on and a surge of melodrama comes with clunky plot-turns toward the end. Many of these cracks are smoothed over by the luminous cast. There is an easy rapport between characters and every actor brings charisma and joy to their role. Adewunmi captures our sympathies as a cornered man who never loses his goodness. Taitte’s Norman is winning, so is Rita Bernard-Shaw as Gwyneth’s daughter, Connie, who has a beautiful singing voice, while Ifan Huw Dafydd oozes natural ebullience as her friend Patsy. Rosie Ekenna, as the youngest daughter, Georgie, is splendid in her kookiness while Parish plays the matriarch with a fabulous mix of steel and softness. Together they breathe humanity into every line, and make this play sing.