In crowded, expensive London, a cash-strapped couple open their small apartment to a third person. Somehow, the new addition makes the flat seem bigger, not smaller. But before long, their collective future is thrown into doubt. Can they find a new way to love? Will the world let them?

Accomplished chef, Gemma (Thalissa Teixeira), and her paramedic boyfriend, Kieran (Gary Carr), live together in West London, above Gemma’s café. Kieran and Gemma are madly in love. But unfortunately, love alone can’t pay café suppliers, or London rent. The pair are barely making ends meet, and their different shift patterns mean they’re like ships in the night. To ease some financial pressure, they’ve decided to take in a lodger. Enter Ray (Ariane Labed), a former world-class synchronised swimmer, whose arrival with a Volvo full of baggage commences a modern-day love story that will change the trio’s lives - and how they view relationships - forever.

Funny, adult and tonally chameleonic, this show is a ‘screwball tragedy’ - psychologically truthful, with a British sensibility and a big romantic heart. It is a show about London, now - a city of transition and contradiction, detailed and tough and wonderful. It’s a city that doesn’t wear its romance as obviously as Paris or Venice or Rome but which has its moments. Walking across Hammersmith Bridge at sunset. The view of the city from the top of Primrose Hill. Racing hand-in-hand to catch the last tube home and just making it through the closing doors. Hailing a black cab in rainy, garish Piccadilly Circus. And the people of London - the kind, funny, diverse, opinionated, vivid Londoners.

The characters learn to navigate this new way of loving with compassion and caution. It isn’t experimentation. This is the real thing. It’s not another show about the regrettable mistakes of 20-year-olds, where the stakes are low and passions are high. This is a world of consequences, in which these accidental pioneers have everything to lose.

It is a show about being in your thirties and giving up on your dreams only to find new ones by accident. It’s a show about overcoming trauma. It’s a show about family, both the ones we’re born into and the ones we make. It’s a show about love, about bravery, and following your heart.

Trigonometry is coming to BBC Two on Sunday 15 March.