It's nighttime, and two cops are outside the West Bay Trust bank, writing a parking ticket for an illegally parked SUV. They mention that the bank had just been robbed the day before, but tonight there's no evidence of trouble. As the cops drive away, however, a glimpse inside the bank proves that not everything is as it seems: four people are trapped inside the bank's vault. The unfortunate foursome? Monk, Natalie, Capt. Stottlemeyer and Lt. Disher.
Flashback to a few days earlier: Monk and Natalie are in the kitchen preparing a meal of burnt toast when they see a television report about a bank robbery - at Monk's bank. At the crime scene, Stottlemeyer is taking statements from the employees about the robbery. They tell him that a heavily-accented Russian in a green hooded sweatshirt forced his way in, grabbed all the money, broke into the safety deposit boxes, and then, after shooting one of the tellers, made his getaway with the stolen goods. When Monk and Natalie arrive at the bank and learn that Trudy's bracelet has been stolen from Monk's safety deposit box, Monk vows to track it down.
After reviewing the crime scene and surveillance footage, Monk makes an important discovery: the Russian wasn't working alone. Someone inside the bank helped him block the cameras and escape through the back entrance. Determined to track down the inside man, and Trudy's bracelet, Monk takes a job as the bank's security guard so he can keep an eye on the staff. Pretty soon, he's narrowed the suspects down to one: bank manager Peter Crawley. But when Monk and Natalie visit Crawley's house to investigate, they find an unpleasant surprise in the trunk of Crawley's car: his dead body. They don't find Trudy's bracelet, but a pawn ticket in Crawley's pocket leads them to a local pawnshop. The bracelet is still nowhere to be found, but the pawnbroker does mention something that piques Monk's interest: apparently when Crawley pawned the items, he mentioned still having another "half a box" full of jewelry.
Following this lead, Natalie and Monk return to the bank, where Stottlemeyer and Disher are waiting. Madge, one of the tellers, lets them in to the closed bank, and Monk explains what he's figured out: the money and stolen goods never left the bank. They're still sitting in a safety deposit box in the vault - the one box the robber allegedly didn't open. Madge agrees to open the box to check, and goes to get the key, leaving the others waiting in the vault. While she's gone, Monk makes an intriguing discovery: he finds six toothpicks, one shorter than the others. With that, he announces that they need to leave. Unfortunately, Madge has been watching him via a surveillance camera, and she closes the door of the vault, leaving Monk, Natalie, Stottlemeyer and Disher trapped inside.
With no way to contact the police, Natalie, Stottlemeyer and Disher figure they're stuck in the vault until the rest of the bank staff arrives in the morning. But Monk isn't so hopeful, and he explains why: all the bank employees were in on the heist. The Russian never existed. The six toothpicks were for the six employees: the one who drew the short straw was the one who would have to get shot, to make the robbery look real. All of which means that none of the bank employees will be coming to the rescue of Monk, Natalie, Stottlemeyer and Disher.
Inside the vault, Stottlemeyer manages to open the mystery deposit box, and just as Monk had guessed, the cash in inside - as is Trudy's bracelet. Monk's happy reunion with the bracelet is short-lived, however, because Stottlemeyer needs it to cut through the lock on an electrical box, where he hopes to find a phone line. When he finally manages to get the box open, he doesn't find one. But he does find something else that's useful: the keypad that controls the electronic banner above the bank's door. Using that, he and Disher manage to program a message with a call for help, and all four are ultimately freed. And what does Monk receive as a thank-you from the bank for solving the case? A free toaster, of course.