Unicorn, Mike Bartlett's new play, involves some characters in chairs discussing a sexual threesome. That's the entire show. Polly (Nicola Walker) is a drunken crosspatch who wants to spice up her loveless marriage to Dr Nick (Stephen Mangan) by bringing a blonde lesbian into the bedroom. Nick, a dithering twerp, doesn't care if it happens or not and he lets his gobby wife talk him into it. She's desperate for a bit of girl-on-girl action because she detests straight men (apart from Nick) and she dated women before she got married. It's not clear why Nick puts up with this charmless windbag who treats him like a naughty spaniel and pouts angrily whenever he speaks.

Polly tracks down a gormless poetry student, Kate, and persuades her to join their triangular orgy. This is another puzzle. An attractive youngster like Kate has no reason to facilitate the sexual fantasies of two needy, bickering has-beens. Kate offers many explanations for her behaviour. Too many. She lost her parents early and she yearns for a family. She belongs to a new generation of sexual pioneers. She's a randy hedonist who fears death and wants to seize pleasure while she can. The more reasons she gives, the clearer it becomes that her character makes no sense.

Nick's romantic motives are another mystery. He and Polly are presented as equals in cupid's marketplace but the visual evidence suggests otherwise. Surely Nick could pick up a svelte young medic from the hospital where he works. With his dreamy eyes and dark curly locks, he has the melancholy air of a violin maestro or a philosophising hermit.