Dennison, a married lawyer, was having an affair with judge Jillian Garr. Dennison and Garr were making out in Garr's car when the car was rammed by John Delancy, who had a grudge against Garr after she ruled against him in a divorce case and cost him two-and-a-half million dollars. Dennison jumped out of the car and ran, as he didn't want his affair to be discovered.
Later on, Garr visited Dennison at his home, and they decided to order a pizza. Right before the pizza arrived, the two got into a heated argument which ultimately led to Dennison beating her to death in the bathroom with a baseball bat. The pizza delivery driver, Julio Alvarez, showed up and witnessed Dennison attacking Garr and tried to stop the fight, and Dennison subsequently beat him to death as well.
Dennison realized that the pizzeria knew Juilo's route. If Julio went missing, the police investigation would have focused on his death, not Garr's, and ultimately would hone in on Dennison as a suspect. To divert attention, Dennison needed someone else to be the last person to see Julio alive. To do this, he disguised himself in a baseball cap and sunglasses and drove Julio's car (with Julio's body inside) to the home of Julio's next customer, which happened to be Natalie Teeger. After delivering the pizza, Dennison ditched the car and body in a location where it would be more easily found.
Dennison made a critical error when he accidentally gave Natalie a $50 bill in change. Natalie decided to return the money to him. Once she caught up with his car, she approached and opened the door, and screamed in horror at the sight of Julio's dead body. Natalie didn't realize it at the time but the man in the car was not the same man who delivered the pizza.
Adrian Monk, Leland Stottlemeyer, and Randy Disher arrive, along with an investigation team. During the investigation, Monk starts experiencing flu-like symptoms. Stottlemeyer and Disher are forced to break off when the Commissioner arrives and pulls them off the case, saying that the entire department is being mobilized to search for the missing judge, Jillian Garr, who is also a personal friend of the mayor.
So far, Dennison's plan appears to be working, as the police are too busy investigating the high-profile disappearance of the judge that the murder of the pizza delivery driver is all but forgotten. Natalie, however, refuses to give up, and asks to see the case file. Upon seeing a photo of the victim, Julio, she realizes that it isn't the same man that delivered the pizza. She goes to Pizza Castle to find out where whether he did indeed go over to her house that night, or at the very least, verify that he got all of the receipts for his delivery run.
Natalie retraces Julio Alverez’s route, asking his customers if they saw anything suspicious. The first family did see Julio that night. Then she reaches the second stop, the house of Reggie Dennison. When Dennison answers the door, she notices a bruise under his thumbnail, and suddenly remembers that the pizza guy who came to the door had an identical bruise, and was also wearing a baseball cap and sunglasses. She hurriedly changes her story, claiming to be looking for a lost kitten, then retreats.
When night falls, Natalie sees Dennison leave his house. She sneaks inside and starts looking for clues. She calls Monk (who is sick in bed) on her cell phone and he manages to rally enough to take notice. She sees a law degree on Dennison's wall. Noticing a strong smell of cleaning fluid from the bathroom, she goes in and finds it scrubbed unnaturally clean. Monk tells her to look for traces of blood under the light switch, which “they” always forget. Sure enough, she finds a spot there, and Monk solves the case.
Just then, Dennison returns home, and Natalie drops her cell phone as she backpedals. While she tries to hide and stay silent, Monk starts coughing explosively, drawing to the phone, which he picks up. Unaware that Natalie is no longer on the line, he gives the summation of what happened the night of the murder. Monk (unknowingly) alerts Dennison to the fact that he made not one, but two mistakes: the first is the blood on the light switch (hearing that, Dennison quickly finds it and wipes it away); secondly, his fingerprints are all over the pizza box, which is now at the recycling center.
Natalie grabs Dennison’s phone and tries to call Monk. Switching lines, Monk absently tells her to wait, he’s on the other line... then realizes what has happened. Just then, Dennison finds Natalie and seizes her, telling Monk not to call the police, or he’ll kill her.
Dennison drives her to the recycling center, bound and gagged. As he threatens her to help him find the pizza box, Monk arrives in a cab, still wearing his bathrobe, and tackles Dennison. As they struggle, Natalie runs to a forklift and dumps a load of shredded paper on Dennison, subduing him until Stottlemeyer and Randy arrive.
Unfortunately, they have no evidence to connect him to the murders except the pizza box, which is buried in the huge pile of garbage. Then Monk tells everyone to be quiet: he can hear faint traces of music. It's the musical get well card which Natalie's daugther Julie got him, which he stuck in the recycling bag. Stottlemeyer and Randy locate the package.
Dennison ultimately confessed to both murders and led the cops to Judge Garr's body.