Callie Esterhaus, a young waitress, is getting ready for a big night in a fancy hotel room when her date shows up early. We never see the date's face, but we watch as he leads her out to the balcony, proposes, and then... throws her over the balcony's railing to her death!
Meanwhile, across town Natalie is taking Monk to the theater to see her daughter Julie perform in a play. Julie has a prominent role in the production, so Natalie is filled with nervous excitement, which grows when she sees a well-known theater critic named John Hannigan take a seat in a balcony box. Monk is excited too, but for a different reason. He's gone to the men's room, and he can't stop raving about the immaculately clean facility and the man in charge of it all, a fastidious men's room attendant named Gilson.
On their way home from the show, Monk and Natalie run into Stottlemeyer and Disher, who are at the hotel investigating Callie Esterhaus's death. Monk joins the investigation, and quickly deduces that Callie was proposed to before being thrown off the balcony. Natalie smells distinctive cologne, but no one else notices. Disher arrives with Hannigan's review of Julie's play, and Natalie is furious to read that Hannigan has called Julie's performance "completely forgettable."
The next day, Natalie pays a visit to Hannigan, who coldly refuses to reverse his harsh review. Even his fiancÃ©e, the daughter of Hannigan's boss, is appalled at her future husband's insensitivity. Hannigan's lunch arrives, and his fiancÃ©e bemoans the fact that he eats at the same establishment every day. Natalie smells Hannigan's cologne and hurries off.
Natalie arrives at police headquarters and announces that Callie Esterhaus was killed by John Hannigan. Natalie states her case: Hannigan uses the distinctive cologne she smelled at the crime scene, Callie worked at the restaurant where Hannigan ate every day, and Callie referred to a man in her datebook as "J." - Hannigan's first initial. Callie and Hannigan must have been having an affair, and Hannigan decided to kill Callie before his fiancee found out.
But no one believes Natalie, not even Monk. After all, they saw Hannigan at Julie's play, so he has an airtight alibi. And furthermore, they can't help but wonder if Natalie's anger with Hannigan's review is clouding her judgment. But Natalie is adamant: Hannigan must have found a way to leave the theater during the play to kill Callie, because nobody who actually saw Julie perform would call her "forgettable." To prove her point, Natalie makes Julie sing her song for everyone in the police station.
That night, as Monk does his ironing (of a magazine), his phone rings. It's Natalie, calling from jail. She's been arrested after doing some investigating of her own - in Hannigan's trash. Hannigan doesn't press charges, but Natalie is sure he's "the guy." Determined to prove Hannigan's guilt, Natalie hires a reluctant Monk to pursue her theory.
Monk investigates at the theater, and learns that Hannigan specially requested the box he sat in the night of the play. He also discovers that someone unscrewed the light bulb in the box's exit sign, leaving the area where Hannigan was sitting completely dark when the lights were down. Monk goes to the theater's men's room to visit with Gilson, who tells Monk that the darkened exit leads to a fire escape at the rear of the theater. When Monk tells Gilson about Callie's murder, Gilson is shaken, but he claims he didn't see anything suspicious that night.
Gilson tracks down Hannigan, and asks him about the night of the murder. It turns out Gilson helped Hannigan with a mysterious favor that night, and now Gilson is concerned. Hannigan assures Gilson that he had nothing to do with the murder, and then proceeds to whack Gilson over the head and shove him into his trunk.
The next day, Gilson is in the hospital, alive but comatose. Natalie accuses Hannigan again, but Stottlemeyer and Disher scoff. Then Monk notices Gilson's digital watch. Monk realizes Natalie's been right all along. Hannigan's the guy. Monk remembers seeing "Hannigan" check a digital watch during Julie's play, but it was really Gilson. The real Hannigan uses a pretentious pocket watch. Hannigan convinced Gilson to take his place during the show under the pretense of needing a "cigarette break." In fact, Hannigan used the time to kill Callie Esterhaus. But there's no proof.
Stottlemeyer brings Hannigan into police headquarters for questioning, but it's not about the Callie Esterhaus matter. Instead, Julie enters and claims that Hannigan attacked her. Hannigan swears he's never seen Julie before in his life, and with that, Hannigan has hung himself, because he wrote so vividly about Julie in his review of the play. The police take him away, but Natalie gets the last word: Julie's not so forgettable now! In fact, Julie's talent lands her a spot in Cal Berkeley's theater program.