At the Guinness World Book Museum, a watchman goes on patrol and finds an intruder. The man attacks him and shoves him into some light bulbs, activating the museum exhibits. Finally he throws him over a ledge and onto a swordfish exhibit, impaling him.
Monk is with Natalie waiting to cross the street, and wondering why she complimented him. He wonders what she's up to and she revokes her compliments. When the broken light doesn't change, Monk insists on staying but she crosses the street anyway and leaves him while she gets their checks at the police station. An older woman, Marge Johnson, comes by and notes the walk light is always broken. She insists on crossing and tells Monk to be a pirate. Marge takes his arm and walks him across, and he reluctantly gives in. He offers to carry her bags and they go to meet Natalie at the police station. Stottlemeyer and Disher know about Marge, because she comes in to complain about her noisy neighbor Mr. Keyes, who plays the drums at night. The police can't do anything further until his court date. Monk overhears and comes to Marge's defense, telling her to have the dispatcher forward her call to him so he can take care of it. She touches him, repeatedly, and offers him candy.
The police are called to the museum to investigate the night guard's murder. The curator wants them to leave the corpse on the swordfish for their exhibit. Natalie is pleased to hear that Monk. No money was stolen but Disher notices that an egg eating robot has been stolen. They find hydraulic fluid indicating the robot was dripping. The curator explains that a jealous egg eater broke in two months earlier and vandalized the display. They're interrupted when Marge calls Monk for help: her neighbor is at it again. They're unaware that the neighbor, Keyes, is the museum thief.
Monk visits Keyes and asks him to stop drumming, but Keyes is less than impressed. However, he agrees to take a break. Marge thanks Monk for his help and invites him inside so she can make him lunch. Monk notices she wrote a cook book and Marge talks about her sister who said she couldn't do it. Then she discusses her dead husband, and the song she only plays once a year on his birthday. Monk notices that she had a son and Marge explains that he died when he was 3 and would be Monk's age now. As they sit down to eat, Monk realizes he hasn't washed his hands. As they eat, Marge offers to write a letter asking for Monk's reinstatement and notices he's separating his mixed vegetables. She mixes them back together and he eats them, and then talks about the museum case.
Stottlemeyer and Disher go to see the egg-eating professional, Winston Kasinsky. He denies breaking into the museum and his equally obsessed wife Gloria supports him. Winston claims to have an alibi and when Stottlemeyer proves skeptical, Winston insists on demonstrating by eating 80 eggs.
That night, a masked thief breaks into a jewelry store and the manager, David Elliot, recognizes him as John Keyes despite the mask. Keyes shoots him and leaves with the gems.
Marge visits Monk at his apartment and starts redecorating, and gives him a scarf as a gift. Natalie arrives and is surprised that Marge has been able to get Monk to redecorate. She soon discovers that Marge has already helped Monk with his shopping and cooking. When Marge discovers that Julie is sick, she tells Natalie to go home and Monk agrees. A happy Natalie cautiously makes for the door but Monk insists she go. Once she's gone, Marge shows Monk a newspaper clipping with his name and offers to make a scrapbook for him. Stottlemeyer and Disher arrive to talk to Marge. They know that Keyes worked at the jewelry store before he was fired, and has a record as a thief. However, Keyes has an alibi: Marge called to complain about the noise he was making at the same time the robbery took place. Stottlemeyer believes her and leaves.
Monk meets with Dr. Bell who explains he has a transformative relationship. Bell notices he refers to his real mother as his "other mother" and Monk insists Marge is a better mother than his real mother. However, Monk begins to wonder if Marge has an ulterior motive. Bell says he has to trust people but Monk insists there's always a catch.
Monk goes to the jewelry store and brings Marge along, but starts to snap at Marge. They figure the killer was an inside man and Marge recognizes a piece of jewelry similar to one her husband gave her. She shows him a photo of her husband with the jewelry and Monk notices that her son has a birthmark. He concludes that Keyes is her son and mother and son worked together. Monk explains that Keyes needed an alibi so they set up the whole noisy neighbor report, and Marge involved Monk to support her claim. Marge denies Keyes is her son but Monk angrily insists his theory is true.
At the station, Monk interrogates Marge and demands to know where the jewels are. He throws down the scarf and says he knew she was pretending, but Marge insists that Keyes isn't her dead son. Natalie arrives to inform them that Keyes made bail thanks to his real mother. Marge takes her scarf and leaves.
Disher confirms that Marge's son died, and Monk and Natalie go to Marge's house to apologize. Marge says that she has no son… again. Monk wonders what Marge could possible think of him. As they walk by Keyes' house, Monk notices hydraulic fluid and has Natalie call Stottlemeyer. Monk points out the fluid and they go insides Keyes' garage. They find the egg-eating robot, dressed in Keyes' clothing and holding drumsticks. Monk explains that Keyes deliberately made noise playing the drums, and then put the robot on a timer to play the drums while music played on a boom box, and he was robbing the jewelry store. Marge saw the motions and heard the music, and thought it was Keyes.
Keyes arrives and holds them at gunpoint. He prepares to shoot them but Disher arrives with the warrant. He thinks the robot is the killer but Stottlemeyer and Monk take advantage of the distraction to take down Keyes.
Some time later, Monk visits Marge, who is preparing to move to Seattle to stay with her sister. They both apologize to each other and Marge gives him his scarf back, telling him to stay warm. They hug and she tells him to remember he's a pirate.