Richard Hannay is at a London theatre, attending a demonstration of the remarkable powers of “Mr. Memory”, a man with a photographic memory, when a fight breaks out and a shot is fired. In the ensuing panic, he finds himself holding a frightened Annabella Schmidt, who talks him into taking her back to his flat.
There, she tells him that she is a spy, being chased by assassins out to kill her. She claims to have uncovered a plot to steal vital British military secrets, implemented by a man who is the head of an espionage organization called “The 39 Steps.”
The next day, Hannay wakes up to find her dead, stabbed with a knife. He sneaks out of the flat disguised as a milkman and takes a train to Scotland, where she had told him she was going to find the leader of the espionage group. On the train, he sees the police on his trail. In desperation, he enters a compartment and, in an attempt to escape detection, passionately kisses the sole occupant, the attractive Pamela. She however manages to free herself from his unwanted embrace and betrays him to the law. He jumps from the train and escapes.
He stays the night with a poor older farmer and his young wife who sees in Hannay the dashing, romantic man she longs for. The next morning, he leaves in the farmer’s Sunday coat, and calls at the house the woman had told him of. There he finds the man with part of his finger missing, the seemingly respectable Professor Jordan, who shoots him and mistakenly leaves him for dead.
The fun continues as this frenetic farce careens from place to place and muddle to mess. The conclusion combines mishaps, mistaken identities, and tongue-in-cheek references to everything we like about murder mysteries and film noir detective movies.