Anne Beddingfeld is young, orphaned, and hungry for adventure when she watches a man fall to his death on the platform of a London Underground station. A doctor in a brown suit examines the body, flees, and drops a scrap of paper bearing a cryptic line of figures. Convinced the death is no accident, Anne deciphers the clue and books passage aboard a liner bound for South Africa, determined to chase the mystery wherever it leads. On the long voyage she falls in with a wry, comfort-loving Member of Parliament, his harried secretary, a glamorous society wife who becomes her partner in detection, and a watchful Englishman who seems to be travelling on official business of his own. As the ship steams south, Anne realises she has walked into a tangle of stolen diamonds, double-dealing, and a hunt for an unseen master criminal known only as the Colonel. The story unfolds through Anne's own breathless account alongside the comic, self-serving diary of one fellow passenger, so the reader watches the same events from two very different angles. Moving from English country houses to Cape Town, the Rhodesian veld, and a remote island stronghold, Anne pushes deeper into danger as she tries to work out whom she can trust. The plot threads spies, romance, and a fortune in gems through a journey that keeps shifting under her feet, and Anne proves a sharp and stubborn investigator who refuses to be sent home.