A peaceful river cruise turns into a site of investigation when a series of murders occurs aboard the pleasure steamer Zodiac. Chief Superintendent Roderick Alleyn is tasked with apprehending an elusive international criminal known as 'the Jampot,' who views human life as a mere obstacle to his objectives. The narrative gains its primary tension through the perspective of Troy Alleyn, the Superintendent's wife, who finds herself trapped in close quarters with the killer. As the vessel navigates the quiet waterways of Constable country, the story unfolds as a classic locked-room mystery where the proximity of the passengers heightens the sense of danger and suspicion.

This novel is widely regarded as a quintessential example of the Golden Age detective tradition, showcasing Marsh's ability to blend sophisticated character studies with intricate plotting. Readers often highlight the unique dynamic between Roderick and Troy Alleyn, noting how her artistic intuition provides a necessary contrast to his clinical police methodology. The setting of the Zodiac riverboat is frequently praised for its claustrophobic atmosphere, which effectively traps the characters and forces the tension to a breaking point. The work remains a staple for enthusiasts of traditional British crime fiction who value clever deduction and refined prose over visceral action.