Tribes
Royal Court Theatre, London SW1

Nina Raine's first play was called Rabbit and was characterised by the competitive banter of its young players. Her second, Tribes, could be called Rabbit, too. It is set amid the verbal jousting of a hyper-articulate family that believes no thought should be left unexpressed. In the extreme case of the paterfamilias, the retired academic and writer Christopher, he believes that an emotion does not exist unless it has first been defined by words.

It is a family so obsessed by vocalisation that the ex-girlfriend of the older son, Daniel, is dismissed for having a northern accent and the daughter, Ruth, ridiculed for attempting a career in opera because libretti are always so naff. The household is so verbal that Daniel, a borderline schizophrenic, hears voices in his head. Only the mother, Beth, sometimes holds her tongue, but she is writing a novel and is therefore guilty by word association.

Into this noisy, hermetic tribe - perhaps less engaging than Raine intends - bursts the sound of silence in the form of Sylvia (Michelle Terry), a young woman who is going deaf. That her quietude should prove such a thunderclap is odd, because the family's youngest child, Billy, is also deaf. We see him first at the edge of the stage, silently eating as a storm of words rages beyond his ken. Played by Jacob Casselden, who is deaf himself, Billy is by far the most engaging character onstage from the start. Although his role in the family is at first difficult to make out, it is not hard to believe his relations love him, which is the position that the play comes round to asserting.

Sylvia's arrival threatens the family because she hails from another tribe, that of, as Chris­topher spits it out, "the deaf community". The members of this family, however, fall in love with her, not for her difference but because, to their relief, she is like them: funny, rude about other people, analytic. She persuades them that signing is a valid language and can hold its own aesthetically, even when translating verse. One of the play's grace notes, indeed, is that its signing is so beautiful.

Billy sees Sylvia as the entry point into another tribe and a way out of a family whose main fault may be that no one, after attaining the age of majority, ever leaves it. He moves out and, in the second act, threatens never to speak to the others again unless they learn to sign - a reasonable request, given that Christopher has found time to learn Mandarin. Unhappily, a new tribesman is the last thing that the disillusioned Sylvia is looking for in her life. Billy's infatuation with his new family threatens their relationship, as does Sylvia's infatuation with hers.

The play, superbly directed by Roger Michell, contains many dramatic felicities and one coup. Surtitles are used not only to translate the sign language, but, in one particularly funny scene, to indicate what (unusually) remains unspoken during a row. The coup comes at its end when Billy pulls out his hearing aid and the theatre hums as if the audience has been submerged under water.

Not every element works this well. Daniel's sudden lunge at Sylvia caused a gasp from the audience the night I saw it. It had been signposted, but still seemed to me to lack logic or consequence. Stanley Townsend's domineering Christopher is less warm and less witty (Smoked roe? "It's like being fucked in the face by a crab!") than he needs to be; let us hope the standard "any similarity between" line at the front of the text provides comfort to Raine's father, the poet Craig.

A plot line about Billy's work as a sign interpreter for the police is in itself difficult to interpret, although its point may be simply that all language asks us to join dots and is inherently full of lacunae and false clues. But this is an exhaustingly clever, funny, emotional, politically incorrect play about families and communication, freshly played, especially by the younger cast. Its ending, which so easily could have been sentimental, is moving beyond words - a title, I might add, that Raine did well to avoid.

Andrew Billen is a staff writer for the Times. “Tribes" runs until 13 November. For details see: royalcourttheatre.com