When two young, unemployed friends decide to pose as international detectives, they inadvertently stumble into a high-stakes conspiracy involving missing government documents from the sunken Lusitania. Tommy Beresford and Prudence "Tuppence" Cowley find themselves navigating a dangerous web of Bolshevik agents and master criminals while attempting to locate a mysterious figure known only as Mr. Brown. The narrative follows their transition from bored, penniless youths to amateur sleuths, utilizing a third-person perspective that emphasizes their quick wit and the precarious nature of post-war London. They must rely on their own ingenuity to outmaneuver seasoned spies who are willing to kill to keep the secret plans hidden.

This novel is widely recognized as the debut of Agatha Christie's recurring duo, Tommy and Tuppence, marking a departure from her more traditional Hercule Poirot mysteries. Readers often note the lighter, more adventurous tone compared to her later, darker works, highlighting the charm of the protagonists' youthful energy. The plot is frequently cited for its classic spy-thriller mechanics, which capture the anxieties of the early twentieth-century geopolitical landscape. While some critics point to the narrative's reliance on coincidences, it remains a staple for those interested in the evolution of the detective genre and the early stylistic development of Christie's writing.