Appearances are deceptive. You could easily take Che Walker's second Royal Court play as a standard piece of "in yer face" theatre. But, although it's undeniably visceral, in the end it's a sharp-witted study of sentimentalised violence and the use of language as a form of moral camouflage.

Walker's setting is a grotty, high-rise, north London flat occupied by Deirdra who keeps a crucifix on her wall and whose vocabulary is robustly Catholic. Her peace, however, is rudely shattered by the arrival of a burly, heavy, Joseph, claiming to be the long-lost father of her half-brother, Vincent. It would seem that Vincent has impregnated the half-wit daughter of one of the Camden Calderazzo gang; and Joseph's apparent mission is to protect his errant boy from the murderous wrath of the local mafia.

There's a touch of Pirandello about the initial encounter between Joseph and Deirdra in that we are up against our old friend, the relativity of truth; or at least the unguessable truth about one's relatives.

But, with the arrival of the blood-stained Vincent, Walker's play turns into an acute study of the simultaneous softness and cruelty of the fringe criminal classes. WS Gilbert's cut-throat, who "loves to hear the little brook a-gurgling" has nothing on Joseph who, having broken his son's fingers, goes all gooey about the baby the boy has fathered.

It is Walker's language, however, that really excites: the characters cover base actions with fastidious phrases in a manner reminiscent of Joe Orton. "Vincent, I really think you're being a little exuberant about this," claims Deirdra, as her half-brother threatens to blow his father's brains out or tip him over the apartment's ledge. And when Vincent is accused of having raped the backward Calderazzo daughter, he cries: "I got demons." What Walker pins down precisely is a community with a half-remembered morality and a belief that words can excuse criminal deeds.

Wilson Milam's Theatre Upstairs production shows the same relish for comedic violence he exhibited in The Lieutenant of Inishmore and the three actors exactly convey the characters' muddled morality. Tamzin Outhwaite invests Deirdra with a wondrous mix of abrasive toughness and residual decency as if a sacred heart still beats under her shiny tracksuit.

And Michael Attwell as the crooked patriarch, and Andrew Tiernan as his abusive son cosily suggest that the family that preys together stays together. Walker's plotting may be a bit shaky, but he captures perfectly the gap between words and actions amid the Camden Town gang.

· Until June 7. Box office: 020-7565 5100.