The film starts out with Moochie and his brother, Wilby, creating a missile interceptor in the basement. The Missile breaks a hole in the ceiling and the two are forced to patch it up. After the incident, his dad made him and Wilby give up all their pets.
When he learned his brother turned into a dog, he was delighted, because he always wanted a dog, despite his father's hatred for them. He helped distract his father from learning the truth about Wilby. Wilby told Moochie about his condition, but Moochie preferred his brother as a dog.When his father found Wilby as a dog, his dad grabbed the shotgun and tried to shoot him. Moochie tried to tell his dad that the dog was Wilby but he wouldn't listen.He later tried to find Wilby, but was disappointed to see that Wilby was a human. When Wilby learned Moochie wanted to put a leash and collar on him, he scolded him saying he was through with being a dog. He attended the country club dance, where he helped notify Wilby when he was turning into a dog. He tried to help Wilby escape but was found by Francesca and Buzz.
Moochie later was caught reading a book on dog training instead by his dad. Wilby found Moochie and told him about the spy's plot. Moochie told his dad about the spies but he would't believe him. After Wilby revealed himself to his father, he and Moochie went down to police headquarters to tell them about the spies. When he realized that the meeting with the authorities was getting nowhere, he said he didn't believe his brother was a dog so he could leave, really hurting his bewildered father's feelings.
After being dropped of back home, Moochie climbed into Francesca's house to try to find Wilby. He helped free Wilby and bumped into Buzz on the way out. When his distraught Dad came home, he asked him to follow Wilby, who started to pursue the spies with Buzz's Car.In the end he and his father are hailed as heroes and Francesca lets the Daniels keep Chiffon, thus Moochie finally gets the dog he always wanted.
- The name "Moochie" was a common nickname for Kevin Corcoran. He used that name in some Mickey Mouse Club serials.
- This was the second time Kevin Corcoran played Tommy Kirk's little brother. He previously did so in Old Yeller, and would later do so again in Swiss Family Robinson, Bon Voyage! and Savage Sam.
- This was the first time Kevin Corcoran played Fred MacMurray's son. He would do so again in Bon Voyage!.