The Bowery Boys--Slip, Sach, Bobby, Whitey & Chuck--start their own exterminating service, and get a job which takes them to a spooky old abandoned mansion in the middle of the night. Meeting up with pal Gabe and his new French bride, the boys are tormented by mad scientists who try to convince them the place is haunted and then kidnap Sach in order to place his brain inside a gorilla.
Sach is given a post-hypnotic suggestion that turns him into a championship prizefighter.
Slip (Leo Gorcey), Sach (Huntz Hall), Bobby (Bobby Jordan), Whitey (William Benedict) and Chuck (David Gorcey) unsuccessfully try to sell a dilapidated car to a street cleaner for a fabulous amount, so they can get enough money to save Louie's (Bernard Gorcey) Malt Shop. Sidewalk photographer Cathy Smith (Teala Loring) snaps a pictures of three bank robbers as they are fleeing a robbery but when the Bowery Boys and Cathy realize that Sach is also in the photograph, they break into the photo lab to destroy the negative, which might make the police think Sach was involved in the robbery.
The Bowery Boys head west to clear Louie of an old murder charge that he had killed his gold-mine partner. Sach has the map to the gold mine painted on his back, and Blackjack McCoy has him kidnapped by Indian Joe. Gabe poses as a dangerous gunman, the Klondike Kid, while Slip is in charge of all the remaining loose ends.
Slip invites his cousin Jimmy to stay with his family after he is released from prison. However, Jimmy soon gets mixed up with an auto-theft ring.
Slip, Sach and the rest of the Bowery Boys enter a haunted house, where they engage in slapstick with a gorilla, a robot and a vampire
The Bowery Boys find a lamp that has strange magic powers.
A man wins $50,000 in a card game with gamblers, but is soon found dead and the money missing. Slip and Sach find the money near where the body was discovered, and soon find themselves the target of both the police and the gamblers.
A ghost helps the Bowery Boys capture a gang of crooks led by a mad doctor.
Slip gets fired from his job at a construction company for decking his boss. His sister, who got him a job at the company, is angry with him. Slip manages to get a job with the District Attorney serving warrants, as does Sach. Through his job, Slip finds out that all is not quite kosher at his old construction company, and that his sister may be in danger.
Slip and the gang (Bowery Boys) foil foes of the exiled, incognito king of Truania (Sig Ruman).
When one of the Boys agrees to be hypnotized, he discovers he led a past life in the 1600s as a British tax collector.
The Bowery Boys come to the rescue when a corrupt taxi company puts the squeeze on several independent drivers.
Sach learns that he has inherited a farm in rural hillbilly country, and when he and the Boys arrive there, they find themselves mixed up with a hillbilly clan named Smith who'll shoot anybody named Jones, plus a gang of bank robbers.
While Louie is on vacation, the boys turn The Sweet Shop into an escort service, and soon find a group of beautiful girls as their first clients.
Slip and the gang stray from newspaper work to detective work.
When a cold medicine causes Sach to be able to smell diamonds, he and the rest of the Bowery Boys are induced by a diamond dealer to accompany him to Darkest Africa in search of a legendary cache of them.
Sach receives news that he is the heir to the Terwilliger Debussy Jones fortune. Accompanied by his pal Slip, he arrives at the Jones mansion to review the legal papers needed for him to claim his new fortune. However, Sach and Slip discover that the rightful heir, the young Terwilliger III, is being cheated out of his inheritance by the miscreant duo of Stuyvesant Jones and Clarissa. Sach and Slip, with the help of their fellow Bowery Boys, save the day and restore the heir’s inheritance.
Slip and Sach are working as cleaners in a high rise building. They enter an office to clean it when a messenger hears them use Slip's given name, Terrance Mahoney. The messenger has a letter for "Terrance Mahonoey, Esq." and mistakenly delivers it to Slip. The letter informs Slip that he has inherited a mansion in Long Island. The boys then make their way to the mansion and find that it is inhabited by diamond smugglers. The real owner of the house shows up and helps save the day and defeat the smugglers and gives the boys the house as a reward.
Sach just lost his job as an assistant to a private detective, but he wasn't paid. Slip goes with him down to the detective's office to demand payment, but finds the office empty. A woman enters the office and mistakes Slip for the detective and convinces him to take on a case to find her sister after offering a $50 retainer.