Natalie tries to throw a surprise party for her surprise-hating boss, who is busy trying to solve the murder of a maintenance man. Monk also says that he doesn't like birthday parties because of the one he had decades ago.
The maintance guy was shredded in the machine. In the beginning of the show, they show the guy being pushed in by someone into the shredder. When Monk checks it out, he sees that the lights were off when the person found the maintance guy leading to the thinking of him being murdered.
When Monk goes to the office because Captain Stolllymier said it was urgent, Monk notices that nobody is at the office and that the big pile of plates and cups that were on the table the other day weren't there. He says that Natalie is trying to throw a surprise birthday party. She says that it took a long time to plan it and Monk should go but he hated birthday parties. Then, Natalie said that she would surprise him, but he said that she wouldn't be able to. While investigating the murder, Monk became a little paroniod and thought that Natalie was trying to trick him into a surprise party. At some points she was trying to and he knew so. Anyway, to get more information on the case, they went to the maintance guy's lawyer who was at a place where somebody was selling a solar powered vacuum. The Captain met a girl who didn't tell him her name and the lawyer had a heart attack. Monk thinks he was murdered, but a couple of people took a sip of the same drink and nothing happened to them.
When Monk borrows the solar vacuum, it was as if he fell in love! He filtered the vaccum though when it says not to everywhere on the vacuum and on the booklet. Then he told his story of why he doesn't want another party, where him as his friends watch an entertainer entertain and afterwards, everybody left. His so called friends only came for the entertainment, not for him. Natalie laughs because it sounded silly. He had friends now and that was decades ago! After that, they went to fix the vacuum and Monk noticed that the guy that was selling the vacuum loosened hiis belt and didn't really know how to fix it. He took it for a few days and said he would return it soon.
Natalie finally tricks him into a birthday party by "being chased by a car" When he goes to the party, he was surprised to see it. He enjoyed every bit but during the end. The man who was selling the vacuum was there and put poison in ice cubes to kill Monk. He thought that Monk would figure out the case.
Before Monk took a sip of it, and after the guy he admired when he was small came and he enjoyed the party, he solved the case. The guy who had a heart attack actually got killed by poison in the ice cubes, since his ice cubes were square and others were circlish, just like Monk's now. The ice has to melt which is why the lawyer died only. Another clue was that the guy didn't know how to fix the vacuum because the maintance guy thought of it. This guy just killed them so he could have it to himself. Being correct, that guy was going to jail.
Monk had a good time at the party, and he said it was the best one he ever had.