Now that their children have left the house Charlie and Marion Hume decide to move into an apartment building closer to the office. They rent out their old house to Mr. and Mrs. Thatcher who immediately ask permission to install a burglar alarm.
Rossi gets arrested for hiring the services of a prostitute, while in reality he was just trying to get some information. He is incensed that the young woman was a member of the police force, trying to trick citizens into breaking the law.
Charlie travels to work on public transport, but accidentally leaves his glasses on the bus. As he has a spare pair in the old house, he asks Billie to drive him there. Mr. Thatcher reluctantly lets Charlie and Billie in, but insists on getting the glasses himself. Billie and Charlie have a close look at the furniture, particularly at some Chinese vases. Billie discovers that they are stamped with "property of the Metropolitan Museum of Art". Upon leaving, Charlie is accosted by a former neighbor who complains that strange people keep coming in and out of Charlie's old house.
The next day Billie does some research regarding the Chinese vases. A lady at the museum informs her that occasionally certain vases are lent out to other museums. Billie asks her to look up what happened to the vases she saw in the house. The museum official is willing to look this up, but when she sees where the vases have gone, she refuses to tell Billie anything more.
Marion Hume receives the news that her former gardener and pool man have been fired by Thatcher. This is the final straw for Charlie and he has Lou drive him to his house to tell the Thatchers that they must leave. Thatcher explains that he is no ordinary tenant but part of the police force, trying to figure out which city officials could be bribed in connection with the re-zoning of the Santa Monica Mountains. The entire house has been fitted with cameras and microphones. They have chosen Charlie's house because it has a cellar to store their monitors.
As they leave, Charlie and Lou notice councilman Garvey coming onto the drive. At the offices of the Trib the editors and reporters discuss the ethics of the sting. Rossi, fresh from being a victim of a sting, is dead against any form of entrapment. He checks with some police sources to learn more about the case. To his surprise no-one at the Los Angeles Police Department knows anything about such a sting operation. This means Thatcher might be a blackmailer, rather than a police official.
Charlie and Lou find the house deserted, with all the furniture gone. They are convinced that there was something fishy about the operation. Rossi confronts councilman Garvey with the fact that he was seen at the blackmailer's house. Garvey can show proof that he was elsewhere.
Some time later Lou notices Thatcher on the photograph of an arrest in San Francisco. Charlie and Lou rush to the police with the photograph. There they surprisingly meet Thatcher again, whose real name is Collins. He belongs to a special agency, about which the police was not allowed to talk to Rossi. The entire sting operation was successful: they have caught several officials willing to accept bribes.
The Tribune publishes the story and Rossi is asked to come to Garvey's offices. Garvey tries to explain that he was at the house after all and only accepted the money to find out more about the bribery attempt. Charlie and Marion move back into their old house, but they worry about all the microphones ...