A maintenance man, Bradley Foster, is working late and climbs into a garbage compactor to remove a piece of metal. As he climbs in, a man comes in and turns on the compactor and Foster falls to his death.
Natalie tries to get Monk to the office and asks what he wants for his birthday. Monk hates birthdays, particularly his 50th birthday. They go to the squad room and everyone is hiding in Stottlemeyer's office with a cake. Monk begins to suspect something is going on and interrogates Natalie. When she can't answer, he accuses her of throwing a surprise party. When he threatens to leave, Natalie asks him to play along but he says he can't.
Monk is called to the maintenance man's murder scene and Natalie promises that she's going to throw him a party no matter what. Stottlemeyer and Disher joke about it until Monk points out how painful the man's death must have been. The building manager, Mr. Paul Wellman, comes in and explains how he found the dead man. He insists it must have been an accident but says that he turned on the lights when he came in. Monk points out that someone must have turned off the lights. Wellman says Richard Meckler was also in the building, and Monk wonders what he does.
Meckler is a patent attorney giving a demonstration on a self-cleaning vacuum made by Kurt Pressman. Monk arrives and is in awe of the device. They talk to Meckler, who chews nervously on a pen and who confirms that he was gone by midnight and didn't see anything unusual. Outside, Stottlemeyer is at the bar when he notices an attractive writer, TK Jensen, who for Consumer Currents. He goes over and makes small talk about buying a motorcycle on the magazine's recommendation. Meanwhile, Monk talks to Pressman and expresses his admiration for his work. He wants to buy two and Pressman says he'll put him on the list. Pressman gets a Diet Coke for Meckler, who admits he can't tell what it tastes like. Pressman and Meckler's wife both try it. In the bar, Stottlemeyer tries to guess what TK stands for and admits it'll make it easier for him to track her down. At the reception, Meckler collapses a minute later from a heart attack after drinking his Diet Coke
Later, Monk is at home enjoying his new self-cleaning vacuum and cleaning in Natalie arrives and Monk explains that Stottlemeyer convinced TK to loan him one of the vacuum cleaners. As they go, Natalie says she has to stop at her home to get her cell phone. Monk dials it and it rings in her bag. He figured she was trying to lure him to another surprise party. When she wonders why he can't go to a birthday party, Monk reluctantly agrees to tell her after closing the drapes and turning off the lights. He explains that when he was ten years old, he watched Cowboy Hank, a TV cowboy who was his hero. His mother hired Cowboy Hank to come to Monk's birthday party and he brought a rope to do tricks. He did all his tricks and everyone had a good time... and then everyone left. They didn't stay for cake or anything. Natalie is less than impressed and insists that he has to have a party and promises that she'll surprise him. Monk doesn't believe it and Natalie warns that it is on.
Stottlemeyer and Disher search Foster's home and notice that his computer and tools are missing. Natalie arrives and explains that Monk is outside in the hall because he thinks it's another surprise party. Monk finally comes in and checks every closet and cabinet to see if someone is waiting to jump out. Stottlemeyer assures Monk that Natalie will eventually throw the surprise party, but they have a murder investigation on their hands. Disher explains that Foster went to MIT but then disappeared ten years ago until he showed up as a dead maintenance man. Monk finds some shredded paper and thinks that it's confetti. Stottlemeyer finally gets him to focus on he case and Monk notices a chewed pen just like the one Meckler chewed.
At the morgue, Monk keeps checking the covered bodies but only finds corpse. The coroner confirms that Meckler was poisoned with a fast-acting toxin. Disher says he was there and didn't see anything.
Later, Stottlemeyer calls TK back to the building and tells her that they're going to recreate the crime scene. She realizes that he brought him back just to learn more about her but doesn't object. They recreate the crime scene and position Stottlemeyer, Natalie, and Disher as Meckler, his wife, and Disher. Monk poses as Pressman, who carried the Cokes. Pressman and Meckler's wife both drank from the glass, which means there's no way that he could have been poisoned. Disher confirms that Meckler didn't eat anything, and there was no poison on the pen. As Disher theorizes, Stottlemeyer tells TK to just nod her head and say "interesting."
Back at home, Monk has trouble with his new vacuum and can't get it to work. When Monk admits that he cleaned the filter, Natalie points out that the technical manual says repeatedly not to clean the filter, and points to the sign on the vacuum cleaner that says not to clean the filter. Monk puts his vacuum cleaner to bed and asks God to take him instead. However, he realizes that they can have Pressman fix it and prepares to rush off to visit him. When Natalie insists that Monk can't miss his appointment with Dr. Bell, Monk realizes that's where the surprise party is located. Natalie tells him that he's won and he'll be spending all his birthday parties alone from now on. Monk agrees as long as they can fix the vacuum cleaner.
Monk and Natalie go to see Pressman in his garage workshop. The inventor admits that he can't think about anything now that his friend Meckler is dead. Monk expresses his condolences and asks Pressman to fix the vacuum cleaner. He agrees to take a look and asks Monk to find him a screwdriver. While he works, asks about the cases they're working on. Natalie suggests that Monk is close to solving it, and Pressman says it'll take until after Meckler's funeral to fix it.
As they drive, Natalie suggests that she and Julie come over with a birthday cake and have a small celebration. Monk notices that someone is following them and tells Natalie to make a left turn. Stottlemeyer calls and explains that someone put a $20,000 contract on their life. The hit man opens fire and Natalie takes evasive action. She inadvertently goes down a one-way into a workhouse and a security guard stops them. He's gunned down and Natalie and Monk flee into the buildings. Monk is forced to hide in either a dumpster or a port-a-john. Monk would rather choose death but finally goes with the dumpster. The hit men wheel the dumpster away and tell Monk and Natalie to come out. They get out and Monk discovers that he's been delivered to his surprise party. Natalie hugs him and boasts that she got him.
Later, Monk chats with Stottlemeyer and admits that he was surprised. Stottlemeyer almost convinces him that they really shot the guard. Natalie brings in Pressman, who wishes him a happy birthday. As Monk goes to talk with someone else, Pressman takes his and slips away.
Stottlemeyer thanks TK for coming to the party. Meanwhile, Pressman takes Monk's glass outside, removes the ice cubes, and replaces them with some special ones from a container. He then goes back inside and places the glass back where he got it. The partygoers invite him to make a wish and Monk notices a policeman loosening his belt. He remembers that at the garage, Pressman loosened and adjusted what was supposedly his own toolbelt, and didn't know what tool to use on the vacuum cleaner.
Here's what happened: Bradley Foster invented the vacuum cleaner and took his idea to Meckler. Meckler killed Foster and had Pressman pretend to be the inventor. But Pressman got greedy and killed Meckler so he could have all the money. When Pressman points out he drank from the same glass, Monk looks at his own drinks and realizes that his ice cubes are square but everyone else's are round. The poison was in the ice cubes, and Pressman drank from the glass before the ice cubes with the poison melted. Cowboy Hank arrives to lasso Pressman and the police take the killer away.
Later, Cowboy Hank puts on a show of his best tricks and leaves a happy Monk. TK admits to Stottlemeyer that it's never a dull moment and invites her for a normal date. He admits that his brakes are shot and he doesn't want to slow down. TK explains that her first name is Trudy and Stottlemeyer, realizing it won't work with Monk, says they'll stick with TK.
Natalie points out that Monk is actually happy and no one is leaving. He admits it's the best birthday he's ever had... and tells her not to do it again.