Harold Gumbal robs his own jewelry store and delivers the loot to two thugs who have taken his dog hostage. When a security guard shows up, one of the thugs panics and kills him. With Gumbal now a witness to the murder, the shooter turns to Gumbal and kills him, too, in cold blood.
Monk is called in to investigate. Before he can make any progress, a disheveled, second-rate detective named Marty Eels shows up. But today, Marty's detective skills are far from second rate. In fact, he's amazing. He's even better than Monk. He seems to be solving the case by magic. Monk, meanwhile, is convinced that Marty Eels is cheating.
Marty continues to make amazing discoveries. He leads the police to Harold Gumbal's body, the stolen jewels, and he helps them nab one of the thugs, Eddie Dial. With Marty doing so well, Capt. Stottlemeyer lets him take over the case. Marty is quickly becoming a local celebrity, while Monk grows depressed at being upstaged by another detective.
Monk is so shaken that he's considering hanging up his gumshoes. Then, he gets a break. While Monk is on hold during a phone call, he starts a conversation with Natalie. Suddenly, a voice from the phone interrupts them. Apparently, quality control operators can overhear callers on the other end of the phone, even when the callers are on hold. Monk remembers something important: Marty Eels' mother works as a quality control operator for an airline! She must have overheard the thugs talking about their crime while they were on hold waiting to purchase airline tickets to flee the country. Instead of taking the information to the police, she took it to her son, the struggling detective, so he could appear to 'solve' the crime.
Monk and Natalie confront Marty, who challenges them to prove their theory. They can't, and Marty appears to be off the hook. Things change when Vic Blanchard, the thug from the initial murder who's still at large, calls Marty's office. Blanchard has Marty's mother, and he wants Marty to get him his stolen jewels back from the police, or Mrs. Eels will be dead by 8:20.
Desperate, Marty goes to Monk. He needs a real detective to help him find his mother. Monk asks Marty to think about the phone call. Were there any distinctive noises in the background? Marty remembers the sounds of the ocean, and a 'thud' and a 'clang.' Monk thinks' the noises could have come from a nearby beach amusement park. They rush to the park, but find no signs of Mrs. Eels until Monk makes another connection: Blanchard said '8:20,' not '8:00' or '8:30.' High tide is at 8:20! They rush to the beach below the park, and sure enough, Mrs. Eels is submerged in water to her neck. At 8:20, high tide would have drowned her. Marty Eels jumps in the water and heroically rescues his mother from certain death.
But Mrs. Eels is still critical of her son, even after he just saved her life. Monk and Natalie feel bad for Marty, and decide not to tell anybody about Marty's cheating. In the end, Marty Eels gets the key to the city of San Francisco, and retires from his career as a private investigator. Monk's reward is the knowledge that when it comes to solving crimes, he's still number one.