Detective Sergeant Timo Harjunpää takes on a strangely elusive case in this mystery that begins with reports of a nocturnal visitor who tiptoes through apartments in Helsinki. Nothing is stolen or destroyed, but numerous women have awakened to an unknown presence in their bedrooms, only to assume in the light of morning that it was all a dream. At first the police take little notice, and the women themselves begin to doubt their sanity, but as evidence accumulates, the net closes around Tipi, a lock picker for a local burglary ring. Sergeant Harjunpää's big break comes when the suspect falls in love with one of his nighttime victims, yet he falters in his pursuit when he is forced to reassess his private life.