An elderly woman wins a cherry pie at a town raffle. Before she can drive home with her prize, a man approaches, demanding she give him the pie. She refuses, so the man kills her and drives away with her body.
Captain Stottlemeyer and Lieutenant Disher consider the case a routine carjacking, but Monk suspects there may be more to it. However, before Monk can get too involved, he gets a surprising phone call from his brother Ambrose, who wants to discuss a matter of "life or death." The brothers haven't spoken in seven years, ever since Ambrose failed to contact Monk after the death of Monk's wife Trudy. After some prodding by Sharona, Monk agrees to meet with Ambrose, who never leaves his house due to a severe case of agoraphobia. Ambrose, much like his brother Adrian, has a keen eye for crime, and he believes his neighbor Pat Van Ranken has killed his wife.
Ambrose's suspicions were aroused a couple of nights before, when he heard Van Ranken and his wife fighting, followed by four gunshots and a lot of suspicious behavior by Van Ranken. Monk and Sharona pay a visit to Ambrose's neighbor, and Monk notices a series of clues. They decide to follow Van Ranken, who enters a town potato sack race in which he purposely finishes second. Monk and Sharona continue to spy on him as he secretly tears through his prize - a cherry pie.
The next day, Ambrose makes a connection: he remembers a newspaper photo of the murdered elderly woman... holding a cherry pie! Monk and Sharona decide to attend a bingo tournament, where a third cherry pie is being given away. Sure enough, Van Ranken is there, and he wins the pie. Monk stops Van Ranken, calls in Captain Stottlemeyer, and explains what he thinks happened: Van Ranken's wife was baking pies for the town giveaways, there was a fight, and Van Ranken shot her. Unable to find all of the discharged shells, Van Ranken realized that an incriminating shell must have landed in one of the pies. Stottlemeyer allows Sharona to search the third pie, but she finds nothing.
Monk goes to Ambrose with the news, and Ambrose decides to explain why he's been avoiding Monk all these years: the day Trudy died, she was running an errand for Ambrose, so Ambrose has always blamed himself for her death. Monk tells Ambrose it wasn't his fault, and they make up.
Later, Monk figures out where the missing shell must be: it landed in a bag of flour that Van Ranken's wife had borrowed from Ambrose... and that Van Ranken has since returned! Monk races to Ambrose's house, but Van Ranken is already there, in the process of setting Ambrose's house on fire after being unable to force his way inside. The police arrive in time to capture Van Ranken, but Ambrose is still trapped inside, unable to leave. Monk dashes in, heroically rescuing his brother. The case is solved, and the Monk brothers are finally reconciled after seven long years.