Arising from their graves in the Summerfield cemetery, the ghosts of Jonathan Q. Gildersleeve and Randolph Q. Gildersleeve spot a present-day newspaper headline announcing the candidacy of their relative, Throckmorton P. Gildersleeve, for the post of police commissioner. The two apparitions decide to boost their relative's campaign by having him expose the diabolical experiments being conducted by Dr. John Wells and his assistant, Henry Lennox. To lead Gildy to the deserted house in which Wells is conducting his experiments, the ghosts free the scientist's gorilla and take him to Gildy's house. In addition to his long-departed relatives, Gildy's campaign is also supported by love-sick newspaper columnist Harriet Morgan and Leroy Forrester, Gildy's nephew, who plans to woo the animal-lover vote by dressing up as a gorilla. After Gildy vetoes his nephew's scheme, the escaped gorilla enters the Gildersleeve kitchen and begins to feast, and Gildy thinks that the animal is his hungry nephew until he encounters the boy in the hallway. When Gildy returns to the kitchen with Leroy, his niece, Margie Forrester, his housekeeper, Birdie and his friend, pharmacist J. W. Peavy, the beast has gone. Gildy insists on notifying the police about his uninvited visitor, but his story is discounted as a publicity stunt by Haley, the police commissioner and Gildy's opponent. Determined to prove the animal's existence, Gildy convinces Peavy to act as a decoy by dressing in Leroy's gorilla costume. The two men follow the animal's tracks to the deserted house, and as Peavy enters the building to lure the gorilla, Gildy hides behind a post. When Gildy is approached by the real animal, he thinks it is Peavy, and consequently, when Peavy reappears, he mistakes him for the gorilla and knocks him unconscious. Alerted by the commotion outside, Wells and Lennox come out of the house and scoff at Gildy's tale of a stray gorilla. To prove his point, the doctor pulls the mask from Peavy's head, and Gildy then insists that the scientist examine his injured friend. Wells reluctantly invites them into the house, and as he examines Peavy, Marie, the maid, warns Gildy of evil doings. To discredit Gildy and thus keep his experiments secret, Wells confides to Peavy that he thinks his friend is demented. While waiting for Peavy, Gildy sees the gorilla and takes refuge in the bathroom. The beast follows him, but disappears through a secret panel. As Gildy hides in the shower, former chorus girl Terry Vance, the subject of Wells's invisibility experiments, enters the bathroom and disappears. Becoming visible once again, Terry demands a kiss from Gildy, and when he refuses, she chases him under the bed. Gildy's calls for help bring Peavy running to the bedroom, but before he enters, Terry becomes invisible. When Gildy raves about an invisible woman assaulting him, Peavy starts to believe the doctor's diagnosis. Soon after, Harriet, Leroy, Margie, Birdie, Haley and Haley's chauffeur, Chauncey, arrive at the house in search of Gildy. When a violent rainstorm forces them to spend the night, Wells assigns Terry the task of making Gildy look insane. As the others sleep, Gildy, accompanied by Chauncey, searches for the secret panel. The panel opens into a passageway, where they encounter the gorilla. After locking the animal in a cell, Gildy tells Chancey to stand guard while he goes for help. After Gildy leaves, Terry, pretending to be a ghost, threatens to haunt Chauncey unless he denies seeing the gorilla. She then frees the beast, and when Gildy returns with the others, he finds the cell empty, causing Haley to demand that he withdraw from the election. Later, Leroy visits his uncle and suggests that he don the ape costume to lure the real gorilla. Gildy follows his nephew's advice, and after Chauncey alerts Haley about Gildy's disguise, the real gorilla enters Haley's room and is mistaken for Gildy. When the beast becomes agitated, Haley handcuffs him to Chauncey and tells him to wait in the car. After Chauncey tries to take the head off Gildy's "costume", however, he realizes that he is handcuffed to the real beast, which then snaps off the handcuffs and kidnaps Terry. To rescue her, Gildy, still wearing his costume, chases the gorilla into the woods and offers it a bunch of bananas. Back at the house, Marie confesses all, sending the others into the woods in search of Gildy. When they find the gorilla embracing Gildy, they all realize that he has been telling the truth.