At the heart of the novel is this enigma: how could a murder take place inside a locked room that shows no sign of being entered? The novel is also about the rivalry between the detective Frederick Larson and a young investigative journalist, Rouletabille, to solve the case. Larson finds a suspect who is put on trial, only to have him cleared by Rouletabille, who reveals in the most dramatic fashion the identity of the real murderer.