Though you could enter this story from any number of angles, “Trigonometry” starts with Ray (Ariane Labed), a world-class synchronized swimmer who decides to change her life after a jarring accident.

Ariane Labed, Gary Carr, and Thalissa Teixeira star in this BBC import that gently captures the evolution of feelings between a couple and the woman who leases a room in their London flat. Though you ...