It's nighttime at the world-famous Guinness World Record Museum. A security guard is making his rounds, walking through rooms filled with various exhibits. He hears a creaking noise and turns his flashlight on an intruder wheeling a hand truck. Before the guard can grab his walkie-talkie, the intruder lunges at him, shoving him hard - and impaling him on a display of a giant swordfish!
The next day, Monk befriends an older woman, Marge Johnson, while on his way to the police station. She's been dealing with an inconsiderate neighbor who has recently taken up the drums. The police have been to the neighbor's house twice already, but they can't do much more than warn him. Monk takes a personal interest and agrees to help her out the next time it starts up.
Later, as our crew investigates the museum murder, Lt. Disher notices that the only missing item is "Togo The Egg-Eating Robot." It's a mechanical display commemorating the world's egg-eating champion. The only suspect is a jealous egg-eater from Berkeley, who claims he broke Togo's record.
While at the scene, Monk gets a call from Marge. Her neighbor is banging away at the drums again. Monk hurries over and confronts the neighbor, a man named John Keyes. Keyes agrees to stop playing, and Monk goes to Marge's house as the conquering hero. Marge invites him in for something to eat, and the two end up bonding. Marge shows Monk some old pictures of her family, including a son who passed away. Marge even gets Monk to eat his spaghetti without untangling it.
Meanwhile, Stottlemeyer and Disher have the unique pleasure of visiting the house of the rival egg-eater, but it turns out to be a dead end. The egg-eater did write some threatening letters to the museum, but he has a solid alibi for the night of the break-in.
Later that night, a jewelry store manager is closing up his store. He hears a sound at the door and goes to see what's going on. Suddenly, the doorknob turns - someone is opening it with a key! A masked intruder enters and asks for the store's gems, but the manager recognizes his voice. The intruder removes the mask. It's John Keyes, the bad neighbor! The owner offers him all the merchandise and the money in the register, but it's no use. Keyes shoots him, and the manager collapses, dead!
The next morning, Natalie arrives at Monk's apartment to find him and Marge rearranging his living room furniture. In fact, they've already made breakfast and gone shopping. Marge is taking such good care of Monk that they give Natalie the day off. Just then, Stottlemeyer and Disher arrive to ask Marge a few questions about her neighbor, who they've targeted as the number one suspect in the jewelry robbery. There's only one catch - Marge called to complain about his drumming on the night of the robbery at the exact time it was taking place.
With Marge providing John Keyes's alibi for the night of the murder, Monk begins to suspect that something's up. Whenever someone is nice to him, there's always a catch. Perhaps Marge and Keyes are working together somehow. When Marge shows some photos of her family to the detectives, Monk notices a distinctive mole on her late son's face. It's the same birthmark that John Keyes has. That's all the evidence Monk needs. He explodes, and accuses Marge of being in cahoots with Keyes, who Monk claims is really her son. According to Monk, Marge's complaints to the police were staged to serve as an alibi for Keyes, and she only befriended Monk so he would vouch for her.
Monk is grilling Marge when Natalie interrupts to inform him that John Keyes has made bail - and was picked up by his mother. Monk has made a huge mistake. There was no catch to his friendship with Marge.
Monk goes to Marge's house to apologize, but the trip proves to be unfruitful - until he notices some tire tracks that match those found at the Guinness Museum crime scene. Monk and Natalie follow the tracks to Keyes's garage, where they find the egg-eating robot from the museum dressed in John Keyes's clothes. Stottlemeyer plugs the robot in and the arms start moving up and down in an "egg-eating" motion. Suddenly Monk figures out what's been going on.
Keyes wanted his neighbors to complain about the drumming from the beginning. On the night of the jewelry store robbery, he dressed up the robot (which he stole from the Guinness Museum), placed it in front of the drum set, and turned on a recording of drums playing. Marge would see the mannequin moving and hear the drumming, and claim that Keyes was home all night. As Monk gives his summation, John Keyes enters and holds everyone at gunpoint. Fortunately, Disher arrives just in time- to shoot the egg-eating robot. However, it distracts Keyes just long enough for Stottlemeyer to tackle Keyes and grab his weapon.
A week later, Monk sees a moving van outside Marge's house. She's moving to Seattle to go live with her sister. Monk takes the opportunity to apologize once more, and Marge accepts it. They both regret what happened, but agree to let the past be the past. Maybe next time someone tries to get close to Monk, he'll be a little more receptive.