Hand in Hand, Hampstead Theatre, London

Simon Block's new play is about two Jewish thirtysomething siblings, living in contemporary, multi-cultural London, who are in the thick of an identity crisis. It opens on a park bench in Primrose Hill, where Ronnie, home after four years in Israel, and having failed to complete a PhD, meets his non-Jewish mate Dan, and expects to return to his old room in Dan's flat.

But things have changed. Dan, with his three-year-old son from a former marriage, and relationship with Ronnie's sister Cass, has other uses for the room – he's hoping Cass will move in. Ronnie (an ill-cast, oddly unlikeable Ben Miles), aware that he'll have to return home to his parents' disappointment, starts sowing seeds of doubt in Dan's mind about Cass's commitment.

At the same time, he throws Cass into confusion over whether her hesitancy towards Dan is due to an aversion to the fact that he isn't Jewish. Slowly, the extent to which Ronnie has been affected by his first-hand experience of the Middle East crisis becomes clear. His anger, confusion and shame over Israel's aggression and Palestinian oppression is directed at his parents' conservatism, his sister's complacency and his mate's broken promises – anyone but himself.

Block is a funny writer, his dialogue smart and sassy. But there's too much of it. Most of Gemma Bodinetz's static production is taken up with Rebecca Egan's Cass theorising with her mate Lou over whether Dan loves her, or arguing with Ronnie over their parents' unswervingly loyal position on Israel. Block's anxious desire to write about the generation gaps in the Jewish Diaspora, and its myopic distance from modern Israel in a way that reflects cosmopolitan reality, produces a London vernacular so savvy it ends up sounding implausible.

He is better at throwing witty glances at gender relations than dramatising personal crises of faith. By the same token, though, his concentration on Dan and Cass's emotional issues sidelines the more interesting subject of Ronnie's divided religious loyalties. Tensions climax at Dan's roof party around the time of the eclipse in 1999, when Cass's glamorous boss Helen tests Dan's fidelity, yet the roots of Ronnie's self-hatred and self-pity are revealed far too late. Block's attempts to align the theme of conflicted loyalties in love with conflicted loyalties in faith merely makes a farce of the first, and oversimplifies the latter.

To 16 March (020-7722 9301)