Florian Zeller’s play was raved about on its British première at the Ustinov on Bath this time last year, and again on its first London transfer to the Tricycle in May. I see no reason why the raves should abate now it has deservedly hit the West End. It is terrifying and brilliant in its portrait of Alzheimer’s from the inside.

In James Macdonald’s flawless production, we identify with 80-year-old André as he grumbles about a “stolen” watch that he has merely mislaid and is part-endured but also part-patronised by his daughter Anne. We share his disorientation when a man turns up in his apartment declaring himself to be Anne’s husband of ten years, since she had been a divorcée only a few minutes ago. And when “Anne” returns with an entirely different face, we find it as hard as André to keep a dependable grip on anything. Scene follows scene with changes of face, biographical data and even furniture; sometimes a strand shows some consistency, only to be snapped by a difference of detail. The nature of theatre itself puts us inside the confusion, naturally empathising as André grows quieter and sadder, neglecting even to change out of his pyjamas. Piano music between scenes is similarly discontinuous, with skips, reverses and intrusive sine tones. Miriam Buether’s set is half-distanced from us behind a rectangular cut-out frame. Nothing is reliable in this all-too-plausible portrait of a mind and a world at odds with each other.

Actually, two things are reliable. Christopher Hampton’s translation is as deft as his work from the French usually is. His rendering of Yasmina Reza’s Art was first seen on this stage; The Father is Reza for grown-ups, with both a heart and a mind. The other linchpin is Kenneth Cranham’s performance as André, which begins enticingly to draw us into this reality with him and then keeps us on the journey, somehow growing more towering even as he subsides into morose, bewildered silence. Anyone who has ever entertained consideration of such a condition either in themselves or another will be at once gripped and shaken by this remarkable play. I repeat: terrifying and brilliant.

Written for the Financial Times.