Autumn in a seaside town, and four young girls are found strangled. Each body bears the 'signature' of a killer whom the media soon dub the Monster, and Detective Chief Superintendent Maurice Kendrick is driven by his sense of outrage into obsessive pursuit of the murderer. A fifth killing, gruesomely different from the others, drives his friend Humphrey Barnes into an obsession, too -- to disprove his fear that a beloved member of the victim's family may know something about the murders. Another death, and the ordeal of a girl who escapes, solve the riddle of the Monster, but this is just the beginning of the separate efforts of the two friends to get to the truth of the fifth murder -- Kendrick through orthodox police methods, Humphrey through his tortured contacts with the victim's unhappy family. Since Kendrick's wife left him Humphrey has become his confidant, but he keeps his unofficial ideas to himself as he listens reluctantly to the progress of the Chief Superintendent's official investigations. This time his intuition is not at the service of the police.
It is a race for the truth which only Humphrey knows they are running, and for one of them the final grim revelation represents a hollow victory.