Detaining Justice, Tricycle Theatre, London

Justice Ncube is a young Zimbabwean who, after entering this country on a fake passport, is being held in a British detention centre. He was a member of the MDC, the party in opposition to Mugabe, but now, with the purported sharing of power, there is technically no reason why he should not be deported back to his homeland. Indeed, isn't that precisely where he should be – fighting the good fight? Justice does not see it that way. The set up in Zimbabwe is a con and he will be killed. His appeal is spearheaded by a quondam celebrity prosecution lawyer who is on the run from his own demons, after the suicide in his last case of a young man he knew to be innocent.

Many dramatists would feel that they had the meat there for a drama of narrow-gauge righteous indignation. But one of the many virtues of Bola Agbaje's wonderful play Detaining Justice is that its generous, questioning, humorous spirit refuses to be confined to a drama of simple dialectics. Presented in a production of pitch-perfect performances and pointed zest by Indhu Rubasingham, the play radiates from the central problem in ways that are richly provocative and often rather politically incorrect.

This sometimes stems from a relevant shift of focus. There are scenes with a trio of station cleaners – Nigerian woman (splendid Cecilia Noble), Ghanaian man, and an intense East European, all of them recent refugees – that are miniature master classes in how to raise issues through an unforced ordinary situation, comically perceived. The East European has bought his identity papers from an English man to whom his weekly pay is directly sent. The situation is desperate because his "owner" is now in some financial embarrassment and is not releasing any of the money to the man who earned it. All of this, though, is hilariously counterpointed by rows between the two blacks who disagree on just about everything to do with how to lie low as an immigrant. There's a wonderful, almost slapstick moment, when the indignant East European declares that he intends to take his grievance to the police and his colleagues back away from such a high-risk proposition as if from a swarm of killer bees.

Peter Brook once said that being a nationality was just a metaphor for the more fundamental business of being human. A great insight, but try persuading border guards and immigration officers of its wisdom. And, of course, for political manageability, you have to set limits. The mistake is to assume that you are obeying some grand metaphysical imperative by so doing. It's on such a self-aggrandising misapprehension that all the repressive and punitive measures of the state depend in this department. Nobody spells that out in Detaining Justice but it is implicit in the nature of the material and the good nature with which Agbaje handles it. It's significant that the official hounding Justice is black – his loyalties as a black British citizen over-riding (in his eyes) his fuzzier loyalties to his race.

The most mechanically constructed of the characters is Justice's troubled defender (stiff Karl Collins), the tortured former star prosecutor whose last case ended so distressingly. But the agonizingly twisted strand whereby his sister (superbly moving Sharon Duncan-Brewster) tries to seek justice for Justice in a corrupt system, performs, whether knowingly or not, heart-twisting variations on Measure for Measure. Strongly recommended.

To 15 December (020 7328 1000; www.tricycle.co.uk)