After young Anne Beddingfeld witnesses an accidental death in a London tube station—and the bizarre behavior of a man in a brown suit who flees the scene—she becomes convinced that foul play is at work. A woman is found murdered the next day and the police show no interest in Anne’s theory that the two incidents are connected.

Spurred by a cryptic note dropped by the man in brown, Anne impulsively uses all her savings to book passage on a cruise ship heading to South Africa. On the voyage she finds herself at the center of a high-stakes game involving stolen diamonds, high society idlers, a mysteriously attractive young man, and a master criminal and his double-crossing minions.