Unicorn review - with Erin Doherty, Stephen Mangan and Nicola Walker

The new play from Mike Bartlett comes with an unusual trigger warning. The show, we are told, "contains explicit content and scenes of a sexual nature, which some audience members may find intriguing".

And with Nicola Walker and Stephen Mangan as a self-proclaimed "sexually bored" couple who bring a younger woman into the fold to spice things up, Unicorn is intriguing indeed for much of its running time.

It's extremely funny, too, as Bartlett touches on bothersome aspects of ageing, like Walker's character Polly musing: "It's not comparing your age with other people that's worrying, it's when you start wondering if you’re older than objects."

Buildings have been torn down around them but Polly, a professional poet, and her husband Nick, an ENT specialist, are still standing - or rather they're treading water in a marriage that's cosy, safe and a bit dull.

Enter Kate (Erin Doherty, best known for playing Princess Anne in The Crown), a sparky 28-year-old student whom Polly finds herself attracted to and ponders if she's just what she and Nick need to spice things up.

Kate is the unicorn of the title, a single person recruited by an established couple to form a throuple.

The first half of Bartlett's play is a dance around possibility, full of side-splitting one-liners perfectly delivered by a twitchy, energised Walker and a droll, reticent Mangan. When he's on form the playwright is a masterful juggler of wit, insight and intelligence, and act one of Unicorn is him at his very best.

The set is sort of ugly; a giant cross-section of one of those Chinese lantern lampshades, rising and falling as needed, under which there's just chairs, a sofa, a bench or a bed, as if designer Miriam Buether is struggling to fill the Garrick Theatre stage.

This chamber piece of a play would be better in a smaller theatre but Walker and Mangan, reunited after starring together in The Split on TV, are big names who can fill a 700-plus seater and they fill the stage with impeccable comic timing as they grapple with the dos and don'ts of a scarily exciting new set-up.

Doherty is a live-wire, but Kate's rants about the state of the world get tiresome and there's no heat under all the verbal sparring.

There's also a lack of focus in the second act, where we reconvene with characters two years later, things take a dark turn and there's a series of short scenes that suggest Bartlett can't quite figure out how to end the story.

As noted in the programme, the show was rushed into the West End without much of a development process and that's a problem because, like Polly and Nick's marriage, it needs some work.

Unicorn is at Garrick Theatre until 26 April.