Monk and Natalie stop by Captain Stottlemeyer's office during his nightly webcam date with his girlfriend, Linda Fusco. Meanwhile, Linda's real estate partner, Sean Corcoran, is out of town showing a house to a young couple. Suddenly, an intruder appears. Without saying a word, the intruder shoots Sean Corcoran dead, but allows the terrified couple to escape with their lives.
The next day, Linda hears the devastating news that her partner has been killed. Stottlemeyer promises to find the killer, and sends Monk to the crime scene to investigate. Monk deduces that the killer had a key to the house, and a witness recalls that the killer was wearing lipstick, so perhaps the perpetrator was a woman.
Later, Linda reveals that Sean Corcoran had a female enemy named Helen Hubbert. Hubbert had been writing Corcoran threatening letters after she became unsatisfied with a house he sold her. However, after Monk finds the same shade of lipstick the killer was wearing in Linda's purse, he has another suspect in mind - Linda Fusco. But there's a problem. Linda was on her webcam date with Stottlemeyer, in her bedroom, thirty miles away, just twenty minutes before the murder took place. How could Linda get from her bedroom to the scene of the crime in twenty minutes?
Monk and Natalie decide to test a theory. They take a motorcycle ride, straight from Linda's house to the crime scene. It's Easy Rider all across San Francisco with Natalie in the driver's seat! But the ride takes them an hour, and Linda would have only had twenty minutes. It's not even close. But while they're at the crime scene, Monk finds a flower on the lawn similar to flowers that Linda was wearing on a Hawaiian lei during the webcam date. Monk is convinced that Linda is the number one suspect - but how could she have done it? And more importantly, how are they going to tell the captain? Monk and Natalie are stumped.
They try to break the news to Stottlemeyer as delicately as they can, but understandably, the captain is furious. He refuses to believe that his girlfriend - the love of his life - is a killer. But when the captain interrogates the only other suspect, Helen Hubbert, and she admits that she doesn't have an alibi, Monk is more convinced than ever that Linda is guilty. The killer spared the couple that was with Sean Corcoran, so Monk reasons that the killer must have had an airtight alibi, and needed witnesses to confirm the exact time of death. So according to Monk, Helen Hubbert is innocent precisely because she doesn't have an alibi. Monk is sure that Linda is the killer, but he needs more proof before he can make an arrest.
While Natalie diverts Linda, Monk snoops around Linda's house, where he finds more evidence: a truck rental business card and a bottle of gun oil. Then, Linda shows up. She lures him up to her bedroom, where Monk fiddles around with a pen that's lying on her desk. Then, Linda attempts the one trick that definitely won't work on Monk - she tries to seduce him! Monk runs from the house in horror. He's in a state of shock - but he's solved the mystery.
The next day, at the police station, Disher is throwing an office luau for the captain and Linda, who have been planning a Hawaiian vacation. During the party, Natalie makes her way into Stottlemeyer's office and switches on his computer. On the screen, live via webcam, is Monk, who's listening to Hawaiian music in what appears to be Linda's bedroom. Linda turns off the computer, but the Hawaiian music can still be heard. Everyone files out of the station and follows the sound to a nearby impound lot, where they come to the source of the music - a box rental truck that was towed from Linda's street the day after the murder. The truck's rear door opens to reveal Monk sitting in an exact replica of Linda's bedroom!
Monk explains what happened. Linda rented the truck and built a replica of her bedroom inside. On the night of Sean Corcoran's murder, she was parked near the house Corcoran was showing. She participated in her regular webcam date with Captain Stottlemeyer from the truck. When the date was over, she walked over to the house and waited for Sean Corcoran to arrive. She had the perfect alibi - it would appear that she was thirty miles away, on a webcam date with a police captain, twenty minutes before the murder took place. How did Monk figure it out? He remembered noticing a pen roll on Linda's desk during the date, but when he was in Linda's actual bedroom, the desk was perfectly level. Linda must have been parked on a hill the day of the murder!
Monk solved the case, but it cost the captain the woman he loved, and almost cost Monk a friend. But in the end, justice was served.