Gemma Foster is a doctor, respected by patients and colleagues for mastery, who has been living with her husband for many years and bringing up their little son Tom. One day, she notices a long blonde hair on her husbands scarf. It gives her a reason to suspect the loving one of infidelity. She secretly tells her friend and colleague Ros about her guess. Doctor Foster start collecting dirt about Simon. After a while, new secrets and surprising details of the husbands life are opened to her. Gemma starts losing the face of a calm and reasonable woman who does her job of a great responsibility.