Monk is at home making a meal and dealing with a spill on his apron when Natalie gets a call about a job. They're meeting with the client, Roderick Brody, the next day. Brody knows Monk from school, and Monk remembers how Brody used to terrorize him in middle school by giving him swirlies. Monk is so distracted he burns his hand and realizes his nemesis is back.
Monk meets with Dr. Bell on short notice and figure that Brody is going to pick up where he left off. Dr. Bell is skeptical and suggests that Monk take the opportunity to resolve the issue from his past. Monk would rather have Dr. Bell sign a note telling Natalie that Monk can't go, but eventually gives in.
Natalie and Monk go to Brody's home where his wife Marilyn greets them. She explains that they had a small wedding and doesn't have any close relatives. Marilyn goes to get him and Brody finally comes downstairs and is glad to see Monk. Before they begin, Monk takes out a note and reads from it saying that he's a broken man because of Roderick's middle school bullying. Brody blows it off and explains that he thinks his wife is having an affair. She told him she was going out of town with some girlfriends a week ago, but he noticed her two days ago at Lafayette Square when she was supposed to be gone. Monk doesn't usually take matrimonial cases but he's eager to make an exception.
Outside, Monk is eager to prove that Marilyn is cheating on Roderick so he can get paid to tell his childhood bully the news. They go to Lafayette Square and try to determine where Marilyn was going to. They check out a cocktail lounge and ask the bartender if he's seen Marilyn. The bartender quickly denies it and even a $1 bribe doesn't change his mind. However, they spot Marilyn in a booth with a man. Monk tries to get a picture on his old camera without success and Natalie convinces a barfly to take a picture of them with Marilyn in the background.
Monk and Natalie take the photo to Brody and explain the man is Douglas Fendle. Brody has no idea who Fendle is and the old film didn't produce a viewable picture. Brody begins to wonder if he imagined the whole thing and insists he needs real proof. He points out that Marilyn bought cruise tickets for a second honeymoon and thinks they're fine. Monk insists that Brody's wife is cheating and Natalie decides he's right. As Brody takes out Monk's fee from the glove compartment, they notice he has a gun but he says he has a permit. He tells Monk that he's dropping the case.
The next day, Monk has Natalie buy him a new camera so he can pursue the case for free. Natalie isn't comfortable with what amounts to stalking but Monk wants to make sure that Brody gets his comeuppance. They're interrupted when Disher calls to tell them that Fendle has been found dead in a hotel. He was an attorney from Chicago and has been stabbed to death. An upbeat Monk thinks that Fendle is responsible and Stottlemeyer notices that he's unusually happy. Monk explains that he was following Marilyn which led to Fendle, and Stottlemeyer figure he's out for revenge. Natalie refuses to tell them their client's name but Monk eagerly volunteers it.
Stottlemeyer brings Brody in for an interview, and he insists that the woman in the bar wasn't Marilyn. Monk warns him that it looks bad but Brody claims he was at home with his wife. Marilyn arrives and says that she can't lie for her husband. She confesses that she told the truth about her affair to Brody and he left, saying he was going to kill Fendle, then returned bloody and gave her a knife to hide. Monk goes outside to enjoy the birds singing and Natalie talks to him. Monk admits he's so happy that he'd like her to do a cartwheel for him, but soon finds himself conflicted. He notices a policeman carrying a gun and remembers that Brody had a gun.
Natalie and Monk go to see Marilyn at her home. She finishes her meeting with her banker and after he leaves, Monk asks her what car he was driving. She confirms the car Brody left in was the car with the gun in the glove compartment. Marilyn suggests that Brody might have changed her mind. Monk concedes she might be right and leaves. Once they're gone, Marilyn picks up an envelope and goes to the basement… where the real Marilyn is tied up. Marilyn's twin sister wants Marilyn to sign the papers but says it isn't necessary and says she needs to speed up her plans.
Monk visits Brody in interrogation and tells him he's going to help him for real now. The detective admits he's been faking it to get revenge. Brody claims that Monk gave back as good as he got because he didn't roll over. Monk is astonished at Brody's claim but pursues the matter. He wonders where Marilyn got the knife and the notices the mirror. Monk remembers that "Marilyn" was at the hotel and killed Fendle. He realizes the real Marilyn was at the hotel while her twin was at the hotel killing Fendle.
Marilyn's sister has given Marilyn tranquilizers and fills a tub with water. She explains that Fendle's firm handled their adoptions and told her about Marilyn. As she pours sea salt into the tub, Disher and Stottlemeyer pull up to where Monk and Natalie are waiting. Disher has confirmed that Marilyn has a twin sister, Patrice Gesner. They don't have a warrant and wait for the officer to arrive with one.
Inside, Patrice explains that she's going to drown Marilyn in the tub the dump her body into the bay. Outside, Monk explains that Patrice pronounced words differently and no longer has bitten nails. They were following Patrice to the bar. Patrice captured Marilyn and would have killed Brody except that the police arrested him first. Patrice was working with Fendle and now that he's dead, is transferring Brody's money to off-shore accounts.
Patrice starts to drown Marilyn but slips and Marilyn screams for help. Stottlemeyer, having probable cause, breaks into the house and they arrive in time to stop Patrice.
Marilyn and Brody are reunited and Monk is less than thrilled. Brody thanks Monk for saving his life and asks if there are no hard feelings. Monk reluctantly agrees, but swears that he can hear the toilet tank of life refilling.