A maintenance man, Bradley Foster, is working late at night as someone breaks into the basement. Foster goes to the basement and dumps garbage into the trash compactor. He discovers that a tire iron is jammed in the works. When he turns off the circuit breaker and climbs down to clear it, the intruder turns the compacter back on and Foster is brutally ripped apart.
Natalie takes Monk to the station house and tries to determine what he wants for his 50th birthday. Monk complains and doesn't want anything for his birthday. He's initially unaware that everyone is hiding in Stottlemeyer's office waiting to yell surprise. Monk soon realizes that the office is empty, someone has cleaned up a big pile of paper plates from the other day, and that everyone is waiting to throw him a surprise party. He refuses to stay and cooperate.
Stottlemeyer calls in Monk to investigate Foster's death. As they arrive, Natalie promises Monk that she's going to throw him a surprise party whether he wants one or not. Monk is not happy. The building manager tells them that he found the body and turned on the lights when he discovered Foster's corpse. Monk points out that someone must have been there and turned off the lights after Foster died: it's murder. The manager tells them there was one other man in the building; Richard Meckler.
Meckler is a patent lawyer and is making a presentation in the building. Inventor Kurt Pressman has built the world's first self-cleaning vacuum cleaner. Monk comes in and is filled with awe at the concept. Disher takes Meckler's statement as the lawyer chews nervously on his pen. At the bar, Stottlemeyer notices an attractive woman, consumer magazine writer TK Jensen, and starts flirting with her. She refuses to tell him what TK stands for.
Monk runs to Pressman and demands to know how the vacuum cleaner works. Pressman directs him to the brochure and is surprised that Monk is praising him so profusely. He promises that Monk will be put on the waiting list. Pressman then approaches Meckler with glass of Coke. He admits he can't tell which one is Meckler's diet Coke and samples it, then has Meckler's wife try it. Meckler finally takes it as Disher watches. Meckler suddenly clutches at his chest and falls down dead, seemingly of a heart attack.
The next day, Monk is lovingly vacuuming with the self-cleaning vacuum when Natalie arrives. He explains that TK let him borrow the vacuum cleaner. As they leave, Natalie says they have to stop at her place to get her cell phone. Monk dials it and it rings in Natalie's purse, and figures the surprise party was at her house. Natalie begs him to come and Monk finally explains why he's so terrified of birthdays. After drawing the drapes and dimming the lights, he explains that when he was ten, his favorite TV show host was Cowboy Hank. Monk's mother threw him a party and had Cowboy Hank come and perform. However, when Cowboy Hank left, so did all the children. None of them stayed for the rest of Monk's party. He's been unhappy at being abandoned ever since. That's it. Natalie doesn’t understand why it was so traumatic for her boss and tells him that he's going to get a birthday party no matter what it takes. She vows to surprise him but Monk insists it isn't possible.
Monk and Natalie go to Foster's apartment where Stottlemeyer and Disher have already found a missing computer and tools. Monk is distracted looking in all the closets to find who is planning to surprise him. Stottlemeyer tells him to focus and that Natalie wouldn't throw a party at a murder victim's residence. Monk isn't convinced. Disher goes over Foster's record. He graduated from MIT, worked for General Electric for 10 years, and then disappeared about five years ago when he turned to drink. Monk goes over the apartment and finds a chewed pen like the one Meckler had at the presentation. They realize that Meckler and Foster are somehow connected.
At the morgue, the coroner reports that they found traces of a fast-acting poison in Meckler's body. As Monk checks the covered corpses to see if anyone is hiding to throw him a surprise party, Disher points out that he was there when Meckler died. Nobody gave Meckler a fast-acting poison while he was watching.
Monk decides to recreate Meckler's murder and Stottlemeyer invites TK to come to the presentation hall. When she wonders why she's there, Stottlemeyer admits he was looking for an excuse to be with her. They recreate the crime and realize that there was no way Meckler could have been poisoned that Disher would have seen. Both Pressman and Meckler's wife sipped Meckler's Diet Coke, and Meckler drank it for five minutes before he died. The pen wasn't poisoned and there was not trace of a dart or needle. Disher suggests a dart on an elastic string but Stottlemeyer isn't convinced.
At home, Monk discovers that his self-cleaning vacuum isn't working. Natalie realizes that he cleaned the filter despite all the warning signs saying not to clean the filter. Monk is in mourning until he realizes they can go to Pressman's workshop to get it repaired. When Natalie reminds him he has an appointment with Dr. Bell and insists he should go, Monk realizes that she's set him up for another surprise party at the office next door. He refuses to go and Natalie insists that if he doesn't, he'll die alone and miserable. Monk is fine with that as long as they get the vacuum cleaner fixed.
At the workshop, Pressman is glad to help and adjusts his tool belt before putting it on. He tries to find the right tool to fix the vacuum cleaner and says that because of Meckler's death, it'll take him a few days to get over it and repair the cleaner. Pressman asks about the case and Monk explains that the two deaths are connected. Natalie assures Pressman that her boss is close to solving the case. Pressman tells Monk he'll call when he has it fixed and Natalie gets Monk out of there.
As they drive home, Monk notices someone following them and tells Natalie to try and get away. Stottlemeyer calls to warn Monk that someone has put out a $20,000 contract to have him killed. They go the wrong way on a one-way drive and find themselves outside a warehouse. A security guard comes out as a black SUV pulls up behind them. When he tries to stop the killers, they shoot him. Monk and Natalie flee down an alleyway and are forced to hide in a dumpster. The killers catch up to them, grab the dumpster, and wheel it away. They then tell Monk and Natalie to get out. Monk discovers that he's been delivered to his surprise party in the warehouse: Natalie set the entire thing up.
Monk congratulates everyone and wonders what happened to the guard. Stottlemeyer briefly claims they had to shot the guard to make it real, and then snickers when Monk wonders if he's telling the truth. Natalie has arrived Monk's hero Pressman to the party. However, when Monk puts his glass down, Pressman picks it up and takes it outside.As TK arrives at Stottlemeyer's invitation, Pressman removes special ice cubes from an insulated container, gets rid of Monk's ice cubes, and puts the tainted ones in.
Everyone sings best wishes to Monk and he prepares to drink from his glass. As he prepares to blow out the candles and make a wishes, he notices a policeman loosening his belt to make room for the cake. Monk remembers that Pressman loosened the tool belt at the workshop, and that he didn't know what tools to use on the vacuum cleaner. He explains that Foster invented the vacuum cleaner. He took the idea to Meckler, but Meckler wanted more money so worked with Pressman to kill Foster. He then had his old friend Pressman pose as the inventor. Pressman got greedy, decided he didn't need a partner, and poisoned Meckler. When Pressman asks how he did it, Monk notices that the ice cubes in his glass are square but everyone else has round ice cubes. The ice cubes have a delayed poison in them. Stottlemeyer takes the glass and an elderly Cowboy Hank arrives to rope Pressman and turn him over to the police.
Later, Cowboy Hank finishes performing and everyone stays for Monk's party. TK admits that it's never a dull moment around Stottlemeyer, and he asks for another chance and two dates in succession. When she wonders if they should slow down, Stottlemeyer admits he's never felt like this and doesn't want to slow down. TK finally tells him her first name: Trudy. Stottlemeyer realizes that's not something Monk should learn and tells her to stick with TK. Natalie points out that everyone is still there for Monk and he admits it's the best birthday he's ever had. He tells her never to do it again.
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