Haunted Child, Royal Court, London (3/5)

Actors are required to do some rum things in the line of duty.

I wonder if, before being offered the role of the father in Joe Penhall's new three-hander at the Royal Court, the excellent Ben Daniels was asked how he felt about pouring a ton of salt into a bucket full of water and then downing the resulting potion in one. Twice. His character, an engineer, has been put on a regime of this noisome purgative by the esoteric religious cult he joined during a recent breakdown when he went AWOL for several days from his distraught wife and disturbed little boy.

It's been quite a year for plays about the desire for some form of transcendental belief in a world of materialistic relativism. There was 13 at the National Theatre and, on this very stage, Alexi Kaye Campbell's The Faith Machine. But one problem with this powerfully unsettling work is that you never for a moment feel the seductive attractions of opting for certainty. You identify with Julie, the anguished, angry wife (beautifully played by Sophie Okenedo) from the moment Daniels's brilliant Douglas resurfaces like a shocking apparition – bedraggled, barefoot, and frighteningly brainwashed. As he spouts the absurd jargon of his new masters and, in a black running gag, gradually reveals just how many ordinary pleasures he is now forced to forgo (“milking” oneself of lust through masturbation is just about OK; the rest of sex verboten), you are left in no doubt that the cult is an outfit run by charlatans who exploit the pain of the mentally ill and rake in their cash through “tithes”.

But if the play is low on ambiguity and devoid of dialectical tension, it nonetheless gets right under your skin – which is a credit both to Penhall's writing and to the finely judged unease of Jeremy Herrin's production. The slightly sinister staircase, the bed-wetting little boy who cannot sleep; the adults whose arguments burden him with conflicted loyalties: these elements strongly reminded me of David Mamet's The Cryptogram. The child here has it a lot worse, though – afflicted, at the start, by ghostly visions of his mysteriously absent father; later, worryingly informed, in bright, born-again fashion, that he's his own grandfather reincarnated; and then paying the various emotional penalties of the instinctive urge to side with this increasingly demented wreck of warped idealism. One of this Christmas's more comprehensive antidotes to panto.