When a pizza deliveryman mistakenly gives Natalie $50 in extra change, Natalie decides to do a good deed. She follows him, intending to return the money, but is shocked to find his car on a deserted street, with the deliveryman beaten to death in the driver's seat.
Stottlemeyer and Disher arrive to assess the situation, but they're soon called away to a bigger case involving a missing Superior Court judge named Jillian Garr. With the authorities distracted, Natalie feels obligated to champion the cause of the dead deliveryman. She goes to Monk for help, but he's feeling sick, and soon finds himself stuck in bed with the flu.
With Monk out of commission, Natalie goes to police headquarters to check on the case of the deliveryman. While there, she makes a startling realization: the man who was killed was not the same man who delivered her pizza! Natalie is now on a mission to get to the bottom of things.
She goes to the pizzeria where the deliveryman worked, and from there, she decides to retrace the route the deliveryman took on the night of his death. This takes her to the home of Reggie Dennison. When Natalie meets Dennison, she recognizes something instantly: he has a black and blue fingernail, just like the man who delivered her pizza on the night of the murder. Natalie knows that Dennison is involved, but she needs to figure out why, and how.
She tries to inform Lt. Disher of her breakthrough, but he and the rest of the force are stationed at Monk's apartment, working on the Judge Garr case. Frustrated with the lack of interest, Natalie takes matters into her own hands and breaks into Dennison's home to look for clues. She calls Monk to update him on her progress, and Monk solves the case! Unfortunately, by the time he starts giving his summation over the phone, Dennison has returned home, taken Natalie hostage, and, unbeknownst to Monk, grabbed Natalie's phone, so while Monk is giving his summation, Dennison is listening the whole time!
Monk explains how Dennison was having an affair with Judge Garr, got into a fight with her, and killed her just as the pizza deliveryman arrived and tried to stop it. Dennison killed the deliveryman, too, and in order to throw the authorities off track, disguised himself and made another delivery. It was Dennison who delivered Natalie's pizza, and his fingerprints on the pizza box would be enough to put him away.
Thanks to Monk's blunder, Dennison has heard the entire summation and knows he needs to get his hands on that pizza box. He takes Natalie to the recycling center where the box should be, but he can't find it. Then ' surprisingly, heroically ' Monk shows up, still sick and still in his bathrobe! Monk battles Dennison until the police arrive, saving Natalie from certain death, and then uses his skills to find the incriminating pizza box.
It's another case solved by Monk, but it would never have been possible without Natalie's courage and persistence.