“You can only despise your own voters for so long. They will judge you as you judge them, measure for measure.”

It’s nearly 50 years since Enoch Powell delivered the infamous Rivers of Blood speech to a Conservative Association meeting in Birmingham.

Often misquoted, the full text of the speech is the centrepiece of this astonishing transfer from Birmingham Repertory Theatre. Powell is played with uncanny and sympathetic naturalism by Ian McDiarmid and his committed, vowel strangled, almost grotesque, delivery reminds us that the speech still has the power to shock.

What Shadows is much more than a re-examination of the 60s colour bar, race relations and racialism. It is a sometimes brutal questioning of what we mean by identity – the identity of an individual, the identity of a race and nation and how we grant admission to outsiders seeking to enter.

The opening scene is strange: a middle-aged woman (Joanna Pearce who also gives fine account as Enoch’s wife) is mending lobster pots. She is joined by Rose Cruickshank (impressively played by Amelia Donkor). They know each other through academia – the former’s career wrecked by the latter’s accusations of racism. They agree to resolve their issues by research – an examination of racism through the medium of Enoch Powell.

This powerful polemic take us to some dark places, difficult dilemmas and bitter exchanges. Does rueing a lost neighbourhood make a person racist? Can a black person be a racist? Do we discover or create who we are?

There are echoes here of the Brexit referendum as a vote against the political establishment: in a chilling moment of insight Powell says: “You can only despise your own voters for so long. They will judge you as you judge them, measure for measure.”

Playwright Chris Hannan’s work takes no sides but exposes the contradictions and confusions of liberalism as falling far short of the certainly most of us crave.

The staging and performances were wonderful: Ameet Chana’s salty, pragmatic Sultan a brilliant pairing with Paula Wilcox’s tragic-comic Grace, and Nicholas le Prevost perfect as journalist Clem Jones.

Four stars