Trigonometry is a love story about three people who are made for each other. In crowded and expensive London, cash-strapped couple Gemm and Kieran open their small apartment to a third person. Somehow, their new addition, Ray, makes the flat seem bigger, not smaller. Gradually, many things become easier, nicer and better with an extra pair of hands.

Everyone is confused after their shared moment in the bathroom the night before. At work, Gemma gets a visit from her estranged father, Ray has a panic attack on a job, and something goes wrong on Kieran's shift.

Gemma and Kieran set a date for their wedding. Ray helps Kieran's recovery and he inspires her to get back in the water. Ray helps Gemma in the cafe. As the wedding nears Ray wonders where she fits in.

It's the day of Gemma and Kieran's wedding, but they're still reeling from Ray's heartfelt declaration the night before. Will the tension between the three come to a head in front of friends and family?

Gemma and Kieran go on honeymoon in Finland, but can't stop thinking about what they've left behind. Ray reconnects with Moi, with calamitous consequences, and starts a new job - will she have space in her life for Gemma and Kieran?

Ray's mum, Mathilde, visits London for Ray's birthday. Mathilde tries to navigate the growing distance between her and her only child. Over a tense birthday dinner, Mathilde observes a new dynamic between Ray and her landlords.

Gemma realises that she's run out of options to save the café. When Ray's solution highlights an inequality in the triad's relationship, the three of them decide to come out to their friends and family.

The triad have never been happier, but then a family tragedy throws everything up in the air. Gemma faces a possibility that she never dreamed of, Kieran's future as a paramedic is called into question and Ray makes a decision.