When an unknown gunman shoots up a barbershop owned by the mob, Monk is called on by Salvatore Lucarelli, the West Coast Godfather, to find the killer and prevent a mob war. Monk initially wants nothing to do with the Lucarelli family, but the Feds convince him to take the job and help bring down the entire organization.
The investigation turns up a witness, Phil Bedard, an employee of the US Mint. He tells Monk that although he didn't see the massacre happen, he did see three men go inside before hearing gunshots fired. Bedard recounts a jacket one of the men was wearing and the symbol on the back, which Monk recognizes as belonging to a Chinese gang. Monk decides it's time to pay a visit to the gang's leader, Jimmy Lu.
After Monk talks to Jimmy Lu, he determines Jimmy and his gang had nothing to do with the murders. He decides to revisit the crime scene because something has been bothering him since the beginning of the case, and he meets with Disher, Stottlemeyer, Sharona and Salvatore Lucarelli at the barbershop. He resurveys the scene, and then spots something in the crime scene photos that only Monk would ever see. He's solved the case!
Monk discovers that the murders weren't mob related at all - they were only made to appear that way to misdirect everyone, including the police, the Lucarelli family, and of course, Monk. The crime scene photos reveal that a large gumball machine is now missing, leading Monk to believe that Phil Bedard, their witness from the US Mint, is responsible. It turns out that the day before the shootings. Bedard stole five double-headed pennies from the Mint, and when he noticed he was being followed by the Mint's security, he slipped into a barbershop to hide the coins in a gumball machine, not knowing the shop was a front for the mob. He found out, however, when he went back to retrieve the pennies, and that's when all hell broke loose.
Bedard unknowingly had set a number of events into motion, including the last and most crucial event: Adrian Monk putting him behind bars.