Spider Kelly, the leader of a gang of young thugs, enters a London jazz club with his underlings The Jinx, The Dancer and Holy Mike. After ordering Jinx to stay behind to guard Spider’s girl friend, The Doll, Spider and the others slip out to rob a warehouse storing cigarettes. On the dance floor, Bert Harris, a young electrician who fancies himself as a dashing criminal, eyes Doll, who dismisses Jinx so that she can dance with Bert. When Spider returns from the robbery and sees his girl friend with Bert, a fight erupts, and Bert escapes in the ensuing melée. The next day, Jinx, an old friend of Bert’s, comes to his flat to show him a newspaper story about the cigarette theft. Scoffing that cigarettes are “small time,” Bert directs Jinx’s attention to the front-page story about a cat burglar who stole a cache of jewels the previous evening. Implying that he is the “cat,” Bert nonchalantly fingers a string of pearls. After swearing Jinx to silence, Bert leaves for M. J. Macey jewelers, where he is to repair the store’s alarm. Jinx proceeds to the gang’s hangout where he shows Spider the story about the cat burglar. When Jinx boasts that he knows the identity of the “cat,” Spider orders him to bring the thief to their hangout that night. Later, police Det. Sgt. Thompson, who suspects that Spider and his boys robbed the warehouse, queries them about the theft, but they give the alibi that they were at the jazz club during the time of the robbery. When Thompson vows that he will arrest them one day, Spider challenges him with a knife, and Thompson responds with a broken bottle. At police headquarters later, the inspector chastises Thompson for resorting to violence. Meanwhile, at the jewelry store, Macey hands Bert a key so that he can adjust the door alarm properly. While working, Bert finds a ring that a salesman has carelessly dropped on the floor and returns it to Macey, who praises the youth for his honesty. After Bert returns home from work, Jinx visits and tells him Spider wants to see him. Adopting a cocky swagger, Bert accompanies Jinx to the hangout and boasts of robbing a bank and a jewelry shop. When the gang scoffs at him, Bert shows them the wiring plan for the alarm box at Macey’s store. Impressed, Spider orders Bert to meet them at midnight that night to rob the store. That evening, Bert takes his girl friend, Rene, to a jazz club owned by his friend Jean, and gives Rene the pearls as a birthday gift while Jean gives her a pair of tickets to the jazz boat cruise the following day. As midnight approaches, Bert, hoping to avoid Spider, insists on leaving, but is greeted by Spider, who is waiting outside in his car. Driving off with Bert, the gang proceeds to the jewelry store where Spider tries to pry open the door with a crowbar. To prevent him from damaging the door, Bert produces the key. Hoping to thwart the robbery, Bert fumbles with the safe’s combination lock, then proclaims that it cannot be opened. Much to Bert’s surprise, however, the door swings open and Spider promptly empties the jewels into a case. To win possession of the jewels, Bert tells Spider that he knows a fence and directs him to Rene’s apartment building. Spider sends Holy Mike and Dancer to accompany Bert, who climbs the stairs to Rene’s apartment and knocks on her door. When she opens the door, Bert pretends that Rene is the fence’s wife and leaves Holy Mike and Dancer waiting in the hallway. Once inside the apartment, Bert jumps out the back window into the alley, intending to go to the store and return the jewels, but once he reaches Macey’s, he learns that the police have already discovered the robbery. Bert takes refuge along the banks of the Thames, and when Jinx finds him there, admits that he is not the cat. When they see Spider and the gang patrolling the area, Bert and Jinx join a procession of merrymakers boarding the jazz boat bound for Margate. They are followed by Spider and his gang, as well as Thompson, who is trailing the gang. After the boat pulls away from the dock, Bert gives Rene the box of jewels to hide in her purse. Spider and his friends then apprehend Bert, and after failing to find the jewels on him, leave Dancer behind to guard Bert while they go after Rene. Once they leave, Jinx knocks Dancer out with a guitar and frees Bert. When Jinx spots Thompson in the bar, he warns Bert that he is a police officer, so Bert decides to capture the gang and turn them over to Thompson. Soon after, Doll finds Bert below deck and proposes that they run away together. When he asks what they will use for money, she hands him the jewel case that Spider has reclaimed from Rene. Just then, Spider appears, and Doll pulls a knife from her purse and slashes him in the face with it. Carrying the jewels, Bert runs into the bar with Holy Mike and Dancer in pursuit. As Bert tosses the case to Rene, who then tosses it to Jinx to prevent the gang from getting it, the revelers pick up Dancer and Holy Mike and throw them overboard. Bert gives the jewels to Thompson, but once the boat docks at Margate, Spider runs ashore, followed by Bert and Thompson, who directs the ticket taker to summon the police. Spider darts into a seaside amusement park and disappears into the House of Ghouls. Amid funhouse mirrors and menacing monsters, Bert and Thompson pursue him. When Spider threatens Thompson with a knife, Bert grabs the weapon from his hand and in the ensuing fight, Thompson knocks Spider out. With Spider and his gang safely stowed aboard the boat in chains, Thompson informs Jinx, Bert and Rene that they are free to go.