It's almost dawn at a suburban "Mega-Mart" retail store, and the store's best employee, Edna Coruthers, arrives early for work. There's a note telling her to go to the loading dock, and when she gets there, a forklift rumbles toward her, knocking over a stack of TV sets. Poor Edna is crushed and killed.
Captain Stottlemeyer and Lieutenant Disher investigate, and the case appears to be a routine industrial accident. But the store's security manager suspects foul play and calls Monk to the scene. It turns out the security manager is Joe Christie, Monk's ex-partner who was kicked off the force years ago for stealing drugs from an evidence room. Christie shows Monk some evidence, but Monk refuses to work with his crooked ex-partner.
A month later, Monk gets to thinking about the evidence Joe Christie gave him: three letters of complaint - supposedly from different customers - against Edna Coruthers. Monk looks at the letters again and makes a key deduction: the stamps on the envelopes came from the same roll. Therefore, all three letters must have come from the same person. Someone was out to get Edna Coruthers.
Monk returns to the Mega-Mart and decides to go undercover as a store employee. He soon focuses in on Jennie Silverman. A former slacker, Jennie is now a model employee, but she is behaving suspiciously: she's constantly being visited by a shady pair of men and she appears to have plans to leave the country. Soon after Monk confronts her, somebody knocks him out and leaves him alone in the store with a killer guard dog. Monk is rescued just in the nick of time by Sharona and Joe Christie.
Grateful to Joe for saving his life, Monk decides to take another look at the case against his ex-partner, and finds a new clue that proves Joe's innocence. With the rift now over between the two ex-partners, they turn their attention toward Jennie Silverman. When Jennie is named "Employee of the Month" for the second time in a row, Monk makes a connection: the previous winner was the murder victim, Edna Coruthers. Did Jennie kill Edna so she could become Employee of the Month?
The answer: yes and no. It turns out that Jennie killed Edna so that she could get access to the Employee of the Month parking spot, which happened to be located in a spot where Jennie and her two flunkies could dig a tunnel into the basement of a neighboring bank. Monk pieces together the crime and, with the help of his old partner, stops the bank robbery.