It is Christmas Eve. A philosophy professor has been dragged from a taxi on her way to celebrate the holidays with her family, and is now held against her will in a grim, nondescript basement.

Across from her, a relentless interrogator (a police officer? hired hand?) intimidates, unsettles and wheedles, in a bid to coax a confession for an unknown offence. Middle class, well-heeled, a tad superior and softly spoken, hers is not the face of a criminal. Or is it?

The police believe they have found incriminating evidence against her – a note claiming responsibility for an undisclosed bombing, to be carried out at the stroke of midnight. She fiercely denies any involvement. Is a terrorist attack imminent or is this a case of state paranoia?

Pulling every trick in the book, the demure academic soon reveals herself to be far more formidable than she first appeared. A left-wing radical with some extreme (if outdated) views who has built a career preaching Guerrilla-style activism, might she have turned theory into violent practice? As midnight looms, time is running out.

Niamh Cusack is deliciously disquieting as Judith, by turns fiery, poker-faced and highly manipulative. As the inquisitor picks out the flaws in her Trotskyist ideology of reform, she appears to be trapped in an intellectual bubble completely divorced from any sense of reality, or empathy (for all her grand claims to loftier, more humane, ideals than the likes of ISIS extremists). Patrick Baladi too is especially redoubtable as her dogged, short-fused - but not unsympathetic - adversary, Thomas. Even he is not immune to the eloquent professor’s florid rhetoric and butter-wouldn’t-melt tack.

Unnervingly played out in real time, the tension becomes stifling as the hands of the clock, in full view, tick inexorably towards the dreaded deadline, steadily tipping the balance of power. This deadly game of cat and mouse is a gripping must-see and a rare feat of theatre.

Christmas Eve runs at the Ustinov Theatre, Bath, until November 18.

- Marion Sauvebois