Frances Cairnes is a woman whose son was killed in a hit and run accident. When the police investigation leads nowhere, she decides to take matters into her own hands.

Posing as a novelist researching a new book, Frances ingratiates herself with the family of George Rattery, the man she believes to be responsible, with a plan to kill him.

Also looking into the matter is detective Nigel Strangeways, who suspects his department of covering up George's involvement in the accident. When he finds that Frances is living with George and his family, Nigel is torn between working to prove George's guilt and stopping Frances' plan for revenge.

Based on the novel by Nicholas Blake.