It's dawn at a construction site, and Ray Galardi, a tough-looking gangster, is meeting with Steve Marriot, an environmental activist. Galardi wants Marriot to withdraw a lawsuit that's holding up Galardi's plans for a strip mall. When Marriot refuses, Galardi smashes him in the head with a lead pipe, killing him.
Later that day, Natalie is driving Monk and Julie down the highway on their way back from one of Julie's field hockey games. A truck cuts Natalie off, and Monk reports it to the trucking company. Minutes later, Natalie is passed by a dump truck driven by the killer, Ray Galardi. Galardi pulls ahead and reaches an isolated stretch of road, where he dumps Steve Marriot's car - with Marriot's dead body inside - onto the highway. The car spins out of control, and causes a huge, multi-car pile-up. Natalie screeches to a halt, injuring her wrist.
With traffic at a standstill, Monk leaves the car to find out what happened. He meets an over-the-hill lawyer named Garrett Price, who joins Monk as he makes his way to the crash site. Police believe that Marriot was driving too fast in his Volkswagen, and that the crash was an accident. But Monk makes an interesting observation: he never saw the Volkswagen pass him. Where did it come from? He also notices that the car left no skid marks, and that its transmission is in neutral. Monk knows this was no accident.
Meanwhile, Ray Galardi has a problem: he confused his cell phone with Steve Marriot's and left it on Marriot's body after he killed him. Galardi returns to the accident site to retrieve his phone, but the police have already locked down the area. Needing access to the crime scene, Galardi kills a paramedic and steals his uniform. Garrett Price, seeing Galardi dressed as a paramedic, asks him to take a look at Natalie's injured wrist. While there, Galardi's cell phone, which really belongs to Steve Marriot, rings. Monk spots the caller I.D., which reads "EGG." Galardi moves on, but Monk is suspicious of this so-called paramedic.
Before Monk can pursue Galardi, he has another assignment: find a bathroom for Julie. Monk spots a tour bus that belongs to the rock band Korn, who allow Julie to use their bathroom. While there, Monk mentions Steve Marriot's name, and one of the band members recognize him as being part of the "Environmental Guerrilla Group." Monk makes a realization: the "Environmental Guerrilla Group"... E.G.G.! Galardi's cell phone must belong to Steve Marriot! Monk sends Julie back to Natalie, and goes looking for Galardi.
Monk finds Galardi, and quickly proves that he is an imposter. But the police don't believe Monk - and lock him up in a police car while Galardi is allowed to go free! Natalie catches up with Monk, who explains what happened: Marriot's car was in the dump truck, and the killer was Ray Galardi, the "paramedic" who's now snooping around the crime scene looking for his cell phone, which got mixed up with Marriot's. With Monk locked up, Natalie decides to search Galardi's dump truck for evidence. Galardi then gets in his dump truck - with Natalie inside! - and begins pursuing an ambulance which contains the incriminating cell phone.
With the situation getting increasingly dangerous, Monk finally gets a break. The police find the body of the dead paramedic, and realize that Monk has been right all along. Monk is set free, and he leads the police on a chase to track down Galardi's dump truck. They catch up to the truck, and Monk heroically keeps Galardi from dumping Natalie on the road by shooting out the truck's hydraulic system. The police cut off the truck and arrest Galardi.
A simple car ride becomes another case solved for the brilliant Adrian Monk.