During a visit with Dr. Kroger, Monk learns that Kroger's son, Troy, has been skipping school and running with a troubled crowd. In fact, at that very moment Troy is skateboarding in a parking lot with his pals Ridley and Pez, and they're about to get into a whole new kind of trouble. The boys come upon a dead man behind the wheel of a car, and the car's backseat is littered with empty bank bags from a recent robbery. There's no money to be found, but Troy comes across a hastily drawn map, and there's a big black "X" marking the spot where the boys assume the stolen money is hidden.
The boys decide to try to find the money in lieu of telling the authorities, so they show up at Monk's house, figuring that the famous detective would be able to help them decipher the cryptic map. They lie and tell Monk that they're studying Treasure Island in English class, and that the map is an extra credit project that they need to solve to keep them from flunking. Natalie is suspicious, but Monk is eager to help Dr. Kroger, and can't resist. Soon, Monk, Natalie, and the three boys are cramming into Troy's car to embark on their bizarre treasure hunt.
Across town, the police have discovered the dead bank robber, whose pockets have already been picked clean by Troy and his pals. Captain Stottlemeyer sees a skateboard ramp and fresh skid marks, and correctly reasons that some kids may have tampered with the scene.
Back in Troy's car, Monk is on a roll - he's figuring out the map clue by clue. Troy talks to Monk about his relationship with his father, and reveals that he secretly wanted to be a psychologist when he was young, and that he wishes he could connect better with his dad.
Later, while our gang of treasure hunters is parked at a rest stop, a cell phone rings from Pez's pocket. He's taken the cell phone off the dead bank robber, and now someone's calling it! Ridley grabs the phone and answers it. Unbeknownst to the boys, the man on the other end of the line is Steven Connelly, manager of the recently robbed bank. Connelly asks if their "little package" has been buried, and Ridley disguises his voice and says that it has, but wants to confirm the exact location. Connelly realizes he's not talking to the phone's true owner, but before Connelly ends the call, Ridley decides to reveal that they have the map and are on the hunt for the money.
Connelly has been talking on the phone to Ridley from the bank, where Stottlemeyer and Disher are questioning him about the robbery. Disher notices a photo of Jack Connelly, a well-known convict, on Steven's wall. Apparently, Steven and Jack Connelly are brothers, but Steven claims to be out of contact with his troubled sibling.
Meanwhile, after an exhausting day on the road, Monk has finally deciphered the map. They head to a quarry, where they find an "X" sticking out of the ground. The kids snap a picture to mark the location, and quickly give Monk and Natalie a ride home so they can return later and dig up their treasure.
The next day, in police headquarters, Monk is reviewing the bank robbery with Stottlemeyer and Disher. He sees that the dead robber's missing ID and cell phone match up perfectly with items he had seen the boys carrying the previous day. He's stunned - suddenly the extra credit project he helped them with doesn't seem so innocent. Monk catches up to the kids at the local convenience store. He explains the trouble they're in, but offers to help them get out of it - after all, he doesn't want to hurt Dr. Kroger.
Later that night, Monk and Troy drive back to the quarry to return the money the boys had found under the giant "X." But something isn't right. The money the boys found was only a small fraction of the total take from the robbery. And why would anyone bury it under a giant "X" in the first place? Monk senses trouble. They race back to Troy's car, but before they can get away, Steven Connelly shows up and uses a bulldozer to bury them under an avalanche of gravel!
Monk and Troy spend the night buried alive inside Troy's car. The only thing keeping Monk from losing his mind is Troy, who calms Monk by getting him to focus on the case. Troy has the dead man's cell phone, and Monk sees that the phone's incoming calls are all from the Connelly brothers. Eureka! Monk explains what happened. The bank robbery was an inside job masterminded by the bank manager, Steven Connelly. There were two bank robbers, Steven's brother Jack, and the dead man the boys found in the car. Jack Connelly was shot during the robbery, and died a few hours later. Steven Connelly needed to quickly hide the body, or the police would know it was an inside job, so he drew a map for the other bank robber, not to show where to bury the money, but to show where to bury his dead brother. Unfortunately, the other bank robber died of a heart attack soon after burying Jack Connelly, and then the kids found the map. Realizing he was in trouble, Connelly put the money and the giant "X" there later, as a decoy for the kids. He figured the kids would find their money and go home.
The crime is solved, but time is running out on Monk and Troy. Fortunately for them, Natalie, Stottlemeyer, and Disher arrive just in the nick of time. Troy hears the car directly on top of them, and in a last ditch effort turns his stereo system up to top volume. Disher hears the music, and Monk and Troy are saved!
The whole experience has a positive effect on Troy, who is now considering going to college to study psychology. In the end, Monk solved the case while helping Dr. Kroger at the same time.