Sixteen-year-old Jacques Rebière is living a humble life in rural France, studying butterflies and frogs by candlelight in his bedroom. Across the Channel, in England, the playful Thomas Midwinter, also sixteen, is enjoying a life of ease, resigned to follow his father's wishes and to pursue a career in medicine.

A fateful seaside meeting four years later sets the two young men on a profound course of friendship and discovery; they will become pioneers in the burgeoning field of psychiatry. But when a female patient at the doctors' Austrian sanatorium becomes dangerously ill, the two men's conflicting diagnoses threaten to divide them and to undermine all their professional achievements.