Lawrence Hammond, a high-powered corporate CEO, is driving his wife to a weekend getaway using the directions given by their car's GPS system when the electronic voice goes haywire, leading them not to a resort hotel but to a deserted lot - a lot where a killer lies in wait.
Monk quickly realizes that Erin, the CEO's trophy wife, was the killer's primary target. Her husband was shot only once, and survived just long enough to give a cryptic clue: "Girls Can't Eat Fifteen Pizzas." No one knows what it means.
Monk pursues the wife angle, and soon discovers that Erin had been having an affair. When Monk sees a sports magazine, he notices that Scott Gregorio, a baseball star, had taken an art class that Erin was also in, and deduces that he was her secret lover.
At the time of the murders, Scott Gregorio was just days away from breaking the single-season homerun record. He confesses to Monk that he loved Erin, and that since her death he has gone into a terrible slump. Monk suspects that Erin might have been killed to keep Scott from breaking the famous record. Scott agrees: just a week ago, someone attacked him and tried to smash his arm. Since then, he's had bodyguards.
Since they've both lost the great loves of their lives, Monk and the baseball star quickly bond, and Monk gives Scott advice about coping. In return, Scott gives baseball tips to Sharona's son, Benjy. Monk is usually completely inept at helping the struggling boy, but once again, in his own way, he comes through.
Monk finally decodes Lawrence Hammond's last words, "Girls Can't Eat Fifteen Pizzas," as the dying man's way of remembering "GCE15P", the license plate of the killer's car. The police soon find the car abandoned, and also discover a security camera took a fuzzy photo of the killer. Monk knows he's seen the face before - but where?
At Benjy's Little League game, Monk agrees to be the umpire. He couldn't be a worse choice for the job, unless you're a fanatic for clean bases and symmetrical scoreboards. Benjy hits a homer, and Captain Stottlemeyer gives him the homerun ball as a souvenir - and that's when Monk has his epiphany. Everyone has seen the killer's face before: he was the guy who caught the record-setting homerun ball three years ago - number 63 - a two million dollar ball that would become worthless if the record was broken again.
Monk and the police close in on Walker Browning, an out-of-work GPS programmer who caught the famous ball. When Browning tries to escape, his dog Toby grabs the ball and runs off - with what will soon become the world's most expensive chew toy.