Stephen Mangan, Nicola Walker and Erin Doherty in Unicorn [Marc Brenner]

MIKE Bartlett’s comedy, Unicorn, is about a middle-aged, middle-class couple, Polly (Nicola Walker) and Nick (Stephen Mangan), who decide they need something to spice up their 25-year-old marriage. That something is another person.

Director James Macdonald sees Unicorn, a play about polyamory, as a natural follow on to Bartlett’s 2009 drama Cock, another three-hander, about sexual identity.

Polly, a poet and teacher, falls for her 28-year-old student Kate (Erin Doherty).

Not wanting to betray her doctor husband, Polly suggests a throuple.

The women are game, but Nick feels uncomfortable – he loves his wife, after all, and they have children. He’s not convinced a threesome is the answer to their mid-life stagnation.

Kate refers to herself as “a unicorn” because it’s rare (and magical) to find a woman willing to share a bed and a marriage with a heterosexual couple. She argues that they are sexual pioneers. Initially, they meet in pairs to discuss their feelings, desires, doubts and logistics, until they inevitably come together as a threesome.

Unicorn is often very funny, gently mocking modern morality, conformity and generational difference, but the play loses momentum in the second half and begins to feel more like a sitcom.

Nevertheless, it’s thrilling to watch this trio of actors at the top of their game. Macdonald’s production is brilliantly cast – the performances are flawless and their timing impeccable.

It’s also beautifully, if intriguingly, staged on Miriam Buether’s canopy-covered set, while Natasha Chivers’ evocative lighting throws up multiple shadows.

Although the story peters out towards the end, the pleasure of Macdonald’s assured production lies in the performances.

Until April 26
www.unicorntheplay.co.uk