Inspector Roderick Alleyn must identify a lethal chameleon known as the 'Jampot' who hides in plain sight among the passengers of a river cruise. As the vessel navigates the scenic waterways of Constable country, Alleyn finds himself surrounded by a cast of eccentric suspects, each concealing their own motives and secrets. The narrative follows a traditional procedural framework, emphasizing Alleyn's methodical observation and psychological acuity as he attempts to unmask a killer capable of assuming any identity. The confined setting of the cruise ship heightens the tension, forcing the investigator to navigate a web of social artifice and shifting loyalties to prevent further violence.

Critics have long lauded Ngaio Marsh for her mastery of the classical puzzle whodunit, frequently placing her in the highest tier of crime fiction writers alongside her contemporaries. The reception of this specific title highlights the effectiveness of its setting and the intricate construction of its suspect list. Readers often appreciate the intellectual rigor of Alleyn's investigative process and the author's ability to maintain suspense within a confined environment. The work is widely regarded as a quintessential example of the golden age detective style, noted for its sharp characterizations and polished prose.