After witnessing a man fall to his death at a London underground station, Anne Beddingfeld finds herself thrust into a dangerous international conspiracy involving stolen diamonds and a mysterious criminal mastermind. Driven by curiosity and a sense of adventure, Anne pursues the truth behind the suspicious doctor who vanished from the scene, despite the escalating threats against her own life. The narrative follows her as she navigates a web of deception that leads from the bustling streets of London to the isolation of a remote mansion. The story unfolds through a classic mystery framework, emphasizing the protagonist's resourcefulness as she confronts forces far more lethal than she initially anticipated.

This novel is widely recognized as one of Agatha Christie's early, high-energy adventure mysteries that deviates from her more traditional village-based puzzles. Readers often note the spirited nature of the protagonist, Anne Beddingfeld, who serves as a precursor to the author's later, more independent female leads. The plot is frequently praised for its brisk pacing and the clever way it weaves together disparate elements like diamond smuggling and political intrigue. While it lacks the intricate locked-room mechanics of her later works, it remains a staple for those who enjoy the blend of suspense and lighthearted adventure characteristic of the era. The work stands as a testament to Christie's ability to craft engaging, fast-moving narratives that keep the reader guessing until the final reveal.