Sumiko Harada, a poverty-stricken Japanese young woman and her father find a key and a letter on the body of a woman accidentally killed on a street in Tokyo. Sumiko's father uses the key to open a train-station locker where he finds a diary written by a sadistic serial killer who believes he is "Akuma," the Japanese mythological devil who can bring misfortune to all who see him.
Sumiko uses the information in the letter and the dead-woman's identity to get a lucrative job at a popular nightclub where she meets Mark Kellner, an American journalist working in Japan. When the killer learns of Sumiko's ruse, Sumiko and Mark are drawn into a web of terror spun by a madman who will use any means, no matter how cruel, to get his diary before it becomes public information.