Throuple might be the current term for three-way romance but it is by no means a new phenomenon. Neither is the idea of a couple saucing up their marriage by bringing in a third party, though that too is given its contemporary moniker – the titular “unicorn” in Mike Bartlett’s play about the travails of one throuple.
It consists of a midlife, middle-class couple, Polly and Nick, whose sex life has lost its fizz. Kate, a gobby young cockney who is a generation apart, might just be the unicorn to bring it back.
The couple is played by Nicola Walker and Stephen Mangan, no strangers to a fictive marriage careering off course (maybe a unicorn might have solved the woes of the couple they played in the hit TV series, The Split), while Erin Doherty’s Kate is the sexual interloper.
Bartlett has dealt with a three-way entanglement before, in his 2009 play Cock, originally directed by James Macdonald, who directs again here. That play was built on a love triad in which a man was romantically caught between his gay partner and a woman.
The triangular desire in Unicorn is an inverse of that in its premise. It is Polly, a teacher and poet, who finds herself drawn to Kate, a student in her class. The opening shows her picking apart her sex life with Nick – not so much vanilla as entirely faded to grey, while flirting with Kate over a drink. So the germ of the throuple idea emanates from their sexual frisson.
It is a clever twist to Cock, but not quite as seductive or convincing. While Polly and Nick are in it to enliven their marriage – though Nick is a reluctant accomplice – the powerful but unconvincing Kate is in it for the words, so she says, quizzically.
What follows is just that: lots of talk and no action – in all senses of that word – and it is oddly atmosphere-less. Conversation spins and bounces from midlife doldrums to the terrible inexorability of ageing to mini critiques of capitalism, modern masculinity, coupledom and nuclear family values. The generation gap is shown up between the couple and Kate, and this services the comedy a little (gen X jokes about She-Ra and nostalgia about pre-digital dating), along with the hand-wringing about the pros and cons of being in a threesome. These are scattered nibblets of thought that undercut the drama’s spring loading and forward movement, especially in the first half. For all the radicalism of the idea, the couple look as if they are in a retro sitcom. When, 45 minutes in, Polly takes Nick’s hand and tries to lead him to the bedroom, he looks like a schoolboy being sent to detention.
Miriam Buether’s semi-circular stage design is focused: a lean arrangement of furniture – mostly a big leather sofa that reeks of TV dinners rather than anything more torrid – against an abstract backdrop. Still, it feels like the stage is too big for the play, the emptiness leeching its intimacy.
There is a pivot in the second half but after all the angsting about alternative romantic configurations, and what they risk in entering into a threesome, there is no exploration of this when they move into the throuple.
Bartlett undoubtedly has a gift for light conversational riffs that carry complex undercurrents. There is some amusing play with words and a few zingy lines but also baggy comedy about a Lycra-clad midlife crisis.
The performances lift it, somewhat. There is a natural ease between Walker and Mangan: she is volubly frustrated, he is hangdog. They are best when the laughter drops off and the intensity is raised but this is not often enough. Doherty brings spirit and comedy. But their relationships are peculiarly devoid of passion, too declarative of their desires. However intimate the conversation becomes, the chemistry in this throuple never quite ignites.
• At the Garrick theatre, London, until 26 April
