Jim McAleavey's house of horrors bulges with invention and endless witty subversions at The MAC

It starts as farce. With the sound of the world’s tiniest gong, or maybe the accidental knocking of a coal scuttle, a girl comes on stage looking for her underwear. This is Kitty (Clare McMahon), whose pants have been warming gently in the armpit of Cait (Helena Bereen), her grandmother.

The two have a close relationship; clearly, if you’re warming someone’s knickers in your armpit you’re close. But this intimacy is also evident from the back and forth, sing-song of the dialogue from the pen of Jimmy McAleavey, winner of the Stewart Parker Award for best new play for his previous effort, The Sign of the Whale.

Cait is a story teller with a finite stock. Her tales are worn down at the edges, eliding, slippery as old steps and, as Kitty joins in with the stories the words twist and mutate, something that Cait is more than happy to join in with. At this point the miscommunication is wilful, a joyous sporting with language: it is a shared game. But, as Kitty’s health deteriorates other, dissenting, damning voices join in.

Ciaran Bagnall’s set design at The MAC in Belfast is astonishingly detailed, with careworn sofas and an old-fashioned fireplace and fully loaded mantelpiece, above which is a beaten metal mirror. Bric-a-brac is stacked and teetering piles of books gather dust, like ones and twos resting on the arm of a record player.

Cait’s house is an ossified womb, cosy and comfortable and brightly lit, and the only home Kitty has properly known. When the sale of her deceased mother’s house goes through, her dreams of leaving her cosy prison and going to London to study theatre become a tantalising possibility.

It would mean a rupture, a break from her grandmother’s oppressive love. Is this the start of Kitty’s journey into hell or is it simply, as her doctor repeatedly says, 'a chemical imbalance in the brain', one of the phrases that chimes throughout the play?

The repetitions of key phrases becomes another marker of the malleability of language, innocent terms made cruel or ironical through the text’s continuously shifting contexts.

As Kitty lies in bed at night she has a premonition 'of something terrible happening' and quickly voices crowd in around her – brutal and withering male voices – crushing and containing her. As she gets sicker they become physical entities, blank-faced, night’s dark agents, crawling from the shadows to continue her torment.

There are some brave costume designs from Elle Kent. True, the swaddled shadow man has a long and mainly ignoble tradition in horror films, but Kent's interpretations work rather brilliantly here. The angel, introduced by a single feather falling from the ceiling, is a feathery loplop, halfway between a plague doctor and Big Bird. It shouldn’t work but it convinces utterly: a shaggy, shuggy horror from the depths of unconsciousness.

That it, like so many of the characters in the play, should seem initially benign but ultimately reveal themselves to Kitty as evil seems particularly apt. McAleavey’s understanding of the way paranoia twists everything into codified attacks is unwavering and truthful. It is one of the many strengths of this astonishing piece of writing.

Beyond the poetry of the language, beyond the brilliant set pieces, beyond the transgressive cruelty of some of the characters in the second half – and it is strong meat – there is an ever-present thread of understanding and of truth. And love too, though love here is used as both a salve and a weapon.

This sense of dislocation continues: Kitty reads a page of Harry Potter as though it is a bickering Beckett monologue. One of Cait’s stories – that of 'Grey Friar’s Bobbie' that she has rebranded as her own – takes on a truly terrible aspect as McAleavey subverts the image of the loyal policeman’s dog into something nightmarish.

Visit’s from her father and boyfriend too, real or imaginary, force her deeper into suffering. It’s horrible to watch, but that is the point.

Unhome is over-long, but when a play is larded with such jewels it must have been hard to shave any bits off. The performances too are exceptional, with Clare McMahon especially fine as poor Kitty: taut, manic, vulnerable, lovable and bereft in turn. It’s an astonishing performance in a truly remarkable play.

McAleavey’s gifts as a writer are manifest. Unhome fairly bulges with invention, his blithe way with dialogue and his endless witty subversions. In the pre-publicity McAleavey mentioned that the play contained a single joke to alleviate the horror, a crack in the crepuscular clouds. I think I must have blinked.

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