It’s the autumn of 1920 and Georgina Somerton is driving to the South Downs to attend a dinner party at her cousin Archie’s. In a sharp bend in the road she notices a single tyre mark that cuts through the verge and disappears over the edge. Fearing the worst, she stops to investigate and finds the body of a motorcyclist. There had been an accident, but that was not the cause of death. He had been murdered. When Georgina returns with the local police the body has disappeared. However, Georgina drove an ambulance during the war and knows very well what she saw.

At Archie’s, Georgina learns that the motorcyclist was to have been one of the guests. Another guest, Major Richard Lawrence, of British Military Intelligence, marks Georgina down as Prime Suspect. Later that night Georgina is in the garage inspecting the other guests’ motorcars searching for clues. She finds only Major Lawrence’s motorcar has any suspicious damage, a broken front headlamp. As she tries to help solve the murder she can’t seem to escape the long shadow of Major Lawrence.

When a second murder occurs, someone’s alibi falls to pieces.