This adventure begins with a body on a train, the word 'Alex' scrawled on the compartment window, and the discovery of a visiting card bearing the name 'Mrs Trevelyan'. More bodies follow, and the two names recur on each occasion.

Paul's attempts to unravel the mystery take him to a psychiatrist's consulting room, a rendezvous in London's dockland, a hotel in Canterbury, and many other places before the denouement in a deserted mill and the Temples' elegant flat in London.