The film starts out with Moochie and Wilby creating a missile interceptor in the basement. The Missile breaks a hole in the ceiling and they are forced to patch it up. After the incident, his dad, Wilson, made him and Wilby give up all their pets.
When Moochie learned Wilby turned into a dog, he was delighted, because he always wanted a dog, despite Wilson's hatred for them. He helped distract Wilson from learning the truth about Wilby. Wilby told Moochie about his condition, but Moochie preferred him as a dog. When Wilson found Wilby as a dog, he grabbed the shotgun and tried to shoot him. Moochie tried to tell Wilson that it was Wilby but he wouldn't listen. He later tried to find Wilby but was disappointed to see that he was a human again. 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 back into a dog. He tried to help him escape but was found by Francesca and Buzz.
Moochie was later caught reading a book about dog training instead of Wilson. Wilby found Moochie and told him about the spy's plot. Moochie told Wilson about the spies but he wouldn't believe him. After Wilby revealed himself to Wilson, 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 Wilby was a dog so he could leave, really hurting Wilson's feelings.
After being taken back home, Moochie climbed into Francesca's house to try to find Wilby. He helped free him and bumped into Buzz on the way out. When Wilson came home, he asked him to follow Wilby, who started to pursue the spies in Buzz's car. In the end, he and Wilson are hailed as heroes and Francesca lets the Daniels keep Chiffon, thus Moochie finally got 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 younger 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!.