Garrick, London; Orange Tree theatre, Richmond
Erin Doherty, Stephen Mangan and Nicola Walker rise to the challenge in Mike Bartlett’s witty dissection of a throuple. And Roger Allam makes a magnificent Churchill in Howard Brenton’s broad-brush latest

What a relief. After a splurge of jetted-in celebrity casting, here are two new plays buoyed up by on-the-spot, on-the-button stage actors. Every Mike Bartlett drama requires a particular dexterity from its performers. Think of Ben Whishaw’s intricacy in the first production of Cock (2009), or Tim Pigott-Smith’s satiric skill with iambic pentameter in King Charles III (2014). In Unicorn, a terrific trio of actors – Erin Doherty, Stephen Mangan and Nicola Walker – are required to hover between lightness and pain. They have to make an explosive proposition seem not only plausible but promising. They need to register, with minimal action, considerable change in the course of two and a half hours. They rise to the challenge.

Despite the twinkling title, there is nothing fey about this drama. Bartlett invites the audience to dismantle the traditional idea of marriage. Unicorn here refers to a person (a rare one) who wants to have sex and perhaps love with an established couple. Doherty, the actor formerly known as Princess Anne in The Crown, is that person: a gleaming, inquisitive, casually knowing young woman who is invited to join, in and out of bed, a comfortably married duo staring at their middle years: “We call it age but really it’s the early symptoms of dying.” Mangan and Walker reunite after The Split: he plays an unsettled ENT doctor, with concertina limbs and lopsided smile; she is a poet and teacher who rushes at everything in a flurry of words and excitable hands.