During a meeting of the Junior G-Men, headed by Penrod Schofield, the club votes to expunge Rodney Bitts. Rodney, who is listening from the loft of the barn where the meeting takes place, lets loose several hornets' nests for revenge. The boys all run out, but Rodney descends slowly, placing confidence in his beekeeper's outfit. Unfortunately, it gets caught on something and he is chased from the barn by a swarm of angry hornets. Rodney rushes to his father, blaming Penrod for his stings. Mr. Schofield, Penrod's father, was about to ask Bitts , his employer for a raise, but Bitts holds Schofield responsible for Penrod's actions and refuses to listen to his request. Returning home that evening, Schofield demands an apology from Penrod and will not listen to his explanation. He forbids Penrod to attend the visiting carnival and locks him in his room. By retrieving the key from the door, "Duke," Penrod's dog, enables Penrod to escape to the carnival. Changing his mind later, Schofield discovers Penrod's disappearance and rushes after him, determined to punish him. When Penrod learns that his father is chasing him, he hides in the basket of a large balloon. Meanwhile, Tex Boyden, a carnival worker, cuts the ropes holding the balloon to get even with his boss, Professor Caligostro. When Penrod does not return that night, the distraught Schofields post a reward. Hearing of this, Caligostro finds Danny Dugan, a double for Penrod, and suggests that he pretend to have amnesia long enough for Caligostro to receive the reward. Danny is so convincing that when Penrod calls from New Mexico, Schofield tells the sheriff that his son has already returned. Only Duke realizes that Danny is not Penrod. The Schofields throw a surprise birthday party for the boy they think is their son. Penrod, who has hitchhiked home, arrives in the middle of the party and, looking through the window, sees his double. Danny sees Penrod at the same time. Feeling guilty because they have been so nice to him, Danny tries to tell the truth to the Schofields, who do not believe him. In the meantime, Penrod has been captured by Tex and Caligostro, who lock him in an abandoned house outside of town. Duke, however, recognizes Penrod and follows the kidnappers' car to the house. Penrod pins a note on Duke and the dog runs back to the clubhouse, where Danny has told the other boys the truth. They follow Duke to the house, capture the criminals and release Penrod. While the boys guard the criminals, Penrod and Danny rush to town to stop Caligostro from obtaining the reward. Caligostro is arrested and the Schofields promise to take care of Danny, too.