Dan Holiday, a mystery novelist who advertises for adventure, receives a letter instructing him to come to 327 N. Greeley St. in Edgemont at exactly 3:30 PM to prevent a murder. The letter is signed Pat Kennedy, but the signature is typed, making Holiday suspicious.

Upon arriving early at 3:20 PM, Holiday finds the apartment unlocked and walks in. He discovers Pat Kennedy dead. Before he can react, a man enters and accuses him of murder. This man turns out to be Lieutenant Johnston from Homicide, who had been tipped off about the crime.

Holiday is arrested, but a mysterious lawyer arrives with a writ of habeas corpus, securing his release. The lawyer refuses to reveal who sent him. Holiday later meets Philip Duval, who claims to have known Pat Kennedy and provides a handwritten letter she had intended to send to Box 13—proving she was afraid for her life.

As Holiday investigates, he realizes the murder was a setup. The original letter he received was typed, but Kennedy’s real letter was handwritten, meaning someone else framed him. The true killer is Lieutenant Johnston, who had a secret relationship with Kennedy and needed to silence her.

Holiday tricks Johnston into confessing by using an inter-office communication box in the police station, ensuring others overhear their conversation. Johnston is arrested, clearing Holiday’s name.