The Munsters are walking through wax statues on exhibit and find a set of Grandpa, Herman, Lily and Eddie. They all pose by the statues, and have Marilyn take a picture just before the museum closes. Once everyone leaves, a man pushes some buttons on a remote control computer, and most of the statues, including Herman and Grandpa, start walking around. A wolfman robot scares a group of gambling men. The Grandpa robot scares a kissing couple out of their car and steals it. The Herman robot frightens a bunch of policemen and steals their uniforms.
The Munsters are at home discussing their upcoming Halloween celebration. Igor can't come, but Aunt Elvira will be flying in. Cousin Phantom of the Opera is already there, always wanting to sing, and sometimes breaking glass because of his high notes. Suddenly the police burst into the house and arrest Herman and Grandpa.
At the police station, Herman and Grandpa don't know why they've been arrested. The witnesses who saw the robots say they were the ones. Glen Boyle, the son of another policeman, questions Marilyn, showing a love interest in her. Marilyn says her uncle and grandfather are very friendly--but at that moment, a bee enters Herman's jacket, causing him to scream like an elephant and unintentionally cause major damage to the station.
Herman and Grandpa are placed in a cell. A rough criminal is brought to the cell and tells Herman (or "Hunky", as he calls him) the bottom bunk is his, but when he sees Herman completely, he takes it back and says if he wants it, it's "Hunky"dory with him. A policeman arrives with soup, but can't open the cell door. Herman bends the bars to help him. The criminal steals the policeman's gun, but Herman takes it from him, and Grandpa takes it from him. Scared, the policeman opens the door. Herman closes it, but Grandpa opens it again, puts the policeman in with the criminal, and closes the door, and Herman bends the bars back in place.
As fugitives, Grandpa and Herman see an illicit exchange of money between the owner of a pizzeria and the people in the museum. They do some sneaking and discover the robots. They pretend to be the robots to find out what's going on. Dr. Diablo, the mastermind behind the robots, plans on using them at midnight on Halloween night to steal artifacts found in an Egyptian tomb. He says his brother stole a valuable necklace and other things from him, and his brother is the mummy. Dr. Diablo and his cohorts think something is wrong with the best robots, Herman and Grandpa, so they electrocute the real Herman and Grandpa, who pass out.
Herman and Grandpa approach a police car. The police want to arrest them and won't listen to the story about the robot. When they call the swat team, Herman and Grandpa give up and run away.
Herman and Grandpa pretend to be women and work in the pizzeria. They get more information on the nefarious plot.
Glen takes Marilyn's advice and investigates the wax museum exhibit, but the people replace the robots with ordinary statues and change the laboratory to an ordinary room. Glen no longer believes the story. He captures Grandpa and Herman, but Grandpa turns into a bat, scaring Glen and making him crash into a fire hydrant. In the chaos, Grandpa and Herman escape again.
Grandpa gets an idea: The mummy can catch Dr. Diablo. In his laboratory in Transylvania, he has a potion that restores life. Herman and Grandpa get in coffins and fly to the old homestead in Transylvania. Igor is shooting flaming arrows at them, but misses. They get in the castle and meet Igor, who says the town wants to get rid of him. Grandpa goes into his old laboratory and finds his potion, while Igor looks out the window at the coming mob and dies. Grandpa realizes he can test if the potion works. He gives some to Igor, who awakens as a young man, after the lights dim and come on again. He tells them that the mob wants the old Igor, and pretends to be part of the mob. Herman and Grandpa return to the United States.
Marilyn hides in the museum and sees the robots in action. She switches costumes with the robot of a scantily-clad torture victim and goes into the laboratory to find out what's going on. Dr. Diablo steps on her foot, and she squeals. He knows that somebody there is not a robot, and says that something's wrong with the costume. Marilyn slaps him. Glen arrives, and Dr. Diablo's assistants put both of them in a bell jar to suffocate.
Herman and the Phantom arrive at the museum and discover Marilyn and Glen in the bell jar. The Phantom offers to entertain them while Herman gets help, but Herman gets an idea. He tells the Phantom to sing a loud and high note. The bell jar shatters and Herman tells Marilyn to take the Phantom home, since the Phantom cannot take her home after such a high note.
It is Halloween night, and there is a public costume party to honor the opening of the sarcophagus. Dr. Diablo and his team are initiating the robots for the tomb raiding. Grandpa turns into a bat and sneaks past their guard. The lights dim, indicating that Grandpa has administered the potion. Dr. Diablo discovers that the robots aren't working, and the man in charge of them tells him that someone stole his computer. Dr. Diablo opens the coffin, and the mummy chokes him. The police arrive and take him away, since Glen has figured out the truth. Glen tells his father that the mummy did the catching. The mummy sits up. "Not exactly," Herman says, just before he takes the mummy costume off. Grandpa asks him if he did not use enough potion. Herman tells him he used too much, as he points to the baby pharaoh.
Everyone returns home, where all the relatives are having their Halloween party. Herman received a replica of an Egyptian crystal, but he doesn't know to whom to give it, since everyone helped. Cousin Phantom sings a welcome home song, and shatters the crystal. Now everyone can have a piece. The whole extended family resumes their Halloween party.