Lucy and Ethel are hanging out the Ricardo's window watching the moving men. Ricky and Fred both want their breakfasts and think the girls are snooping. Due to Ethel's description of the new tenants as a handsome man and a gorgeous blonde, the boys rush over to look. Lucy wants to know everything about the new tenants but Ethel only knows that their last name is O'Brien and that they have a broken lamp because they rented the apartment through an agent.
The girls want to visit them as soon as possible, but Ricky objects. He even makes Lucy promise "not to set foot in the O'Brien apartment". Fred even goes as far as to call information for the O'Brien's phone number but Ethel catches him.
As the O'Briens unpack, they are rehearsing for a television show. The script is about foreign agents coming to America, assuming identities of citizens so they can blow up the capitol. They leave to go out for lunch.
Ethel uses her key to snoop in the O'Brien apartment while they are gone. Lucy also comes by to snoop and keeps her promise by crawling on her knees without her feet touching the floor. While looking at items, they hear Ricky's voice calling for Lucy. He comes downstairs to the O'Brien apartment. Lucy hides in the closet while Ethel and Ricky talk and eventually leave.
With Lucy still in the closet, the O'Briens come home rehearsing their script. Even though there are a few close calls, Lucy is still unnoticed. In the closet, she finds a slipcover for a chair and uses it to get out of the apartment. She pretends to be a chair walking in plain view of the O'Briens. Mr. O'Brien even sits on her. When he stands up Lucy scurries out the door. He then goes to sit on her again, but "the chair" isn't there, leading him to fall on the floor.
Ethel and Ricky are playing gin. Fred comes up looking for the pass key. Lucy rushes in and starts pushing furniture in front of the door. She tries to explain that the O'Briens are enemy agents after them, but Ricky is more upset about Lucy breaking her promise to not a set foot in the O'Brien apartment. She confesses to breaking her promise. Ethel sticks up for Lucy because she was actually in their apartment and heard this. She wants to call the police, yet Ricky thinks she's kidding. The boys go downstairs to turn on the furnace while Ethel tries to calm Lucy down. Lucy does call the police, but when the officer hears the full story, he thinks she's been drinking. The boys return saying that Lucy was right; they heard the O'Briens through the furnace pipe. They start to plan their defense.
Using Ricky's rifles and Fred's two shotguns, the Ricardos and Mertzes defend the apartment. When the phone rings, all four are so scared they shoot the ceiling. The officer comes to check on them and shots are fired, bursting a hole in the door and his cap.
In jail the next morning the officer comes to explain everything; the O'Briens are actors, the dialogue was from a script, and they are moving out. He also informs them that a bullet broke a pipe flooding the Mertz's apartment. Ricky also made the front page of the newspaper because of the shooting. He starts to get angry. Lucy closes the door to the cell insisting she wants to stay inside because it's safer.