Paul Buchanan is the son of antique car dealer Ralph Buchanan and a former classmate of Natalie's in the episode Mr. Monk Is At Your Service.
One day, he discovered that his father had died of a heart attack and panicked. He realized that if Ralph died, then his fortune would go to his wife and Paul's stepmother Camilla Buchanan. This money would then go to her daughters, leaving Paul with next to nothing. He told Camilla that there had been an accident and drove to a deserted road with Ralph in the trunk. He then bludgeoned her with a rock, put Ralph's body at the scene, and attempted to call 911, but failed as there was no signal. He walked down the road and called 911 posing as his father (who he sounded a lot like), before pretending to have a heart attack. This way, it would seem like Camilla died in a car crash before Ralph's heart attack, thus making Paul the next inheritor. His plot worked, and Paul inherited Ralph and Camilla's mansion.
There was still one major problem, however. Edward Stilson, the estate's butler, started to blackmail Paul (although never mentioned, it's implied that Stilson witnessed part of the plot). After giving the latest payoff money as marked bills, Paul was able to learn that Stilson was the one blackmailing him when he found some of the bills in his room. Paul proceeded to confront and kill Stilson, dumping his body in a well in the woods outside his mansion.
One day, Natalie and Adrian Monk were having lunch with Natalie's parents Peggy and Bobby Davenport. They mentioned the case, when Monk and Natalie started to recognize some holes in the story. Ralph was an incredibly pokey driver, so there's no way he would've let Camilla forego wearing a seat-belt. And considering the position of the car and the fact that it was a British model, Camilla should've missed the large rock she apparently hit her head on altogether.
Monk investigated, and accidentally found himself applying for the position of butler under the name Adrian Melville. Eventually, Monk was able to piece together the clues and save Natalie, who was being held at gunpoint by Buchanan.