At the centre of the fainting epidemic are intense and clever Lydia Lamont (Maisie Williams) and admired and rebellious Abbie Mortimer (Florence Pugh), both 16 years old. They carve their initials into a majestic English oak tree by a magical pond and vow never to lose touch. But Lydia already feels that Abbie is drifting away from her and soon her fears are confirmed. Struggling to find solace within the volatile, strange atmosphere of the school and her troubled home-life, Lydia feels driven to discover what is really behind everything that seems wrong. As the fainting escalates Lydia confronts the authority figures around her: her self-absorbed mother, Eileen (Maxine Peake), the unbending and indomitable deputy head Miss Mantel (Greta Scacchi), and the enigmatic and powerful headmistress Miss Alvaro (Monica Dolan). Eventually Lydia’s actions force old secrets to rise to the surface and she finds herself faced with a truth that she never expected.