Late at night in a quiet office building we see Bradley Foster, the building's maintenance man, hard at work. Meanwhile, in the basement utility room, a mysterious intruder is doing some work of his own. He's fiddling with the access panel to the trash compactor. Later, as Foster is finishing up his work, he clicks the compactor on, but it quickly jams. He turns the power off and climbs into the garbage chute. Before he can realize what's happening the intruder steps out of the shadows and switches on the power! Foster is swallowed by the compactor and killed.
The next day at police headquarters, Natalie is trying to find the perfect birthday present for Monk's 50th birthday. Of course, Monk insists that he doesn't want anything - especially not a party, and assures Natalie that she'll never be able to surprise him.
Later on, Monk gets called to the utility room where Bradley Foster was crushed by the compactor. The only other person in the building on the night Foster was killed was Richard Meckler, a patent attorney who recently filed a patent for a self-cleaning vacuum cleaner invented by a man named Kurt Pressman.
Later that day, Monk catches up with Meckler and Pressman at a PR event for the vacuum. They question Meckler, who answers their questions while chewing on a pen cap. Meanwhile, Stottlemeyer flirts with a consumer magazine writer named T.K. Jensen. Then, Meckler collapses! Disher administers CPR, but it's no use. Meckler dies of an apparent heart attack.
Later, while investigating Bradley Foster's loft apartment, Monk and the gang discover some technical journals and tools. It turns out that Foster had studied engineering before a drinking problem forced him into janitorial work. Underneath his desk, they find a plastic ballpoint pen with the end all chewed up - just like Meckler's pen. Meckler had been to Foster's apartment.
The police do a toxicology screening on Meckler which reveals that he did not die of natural causes; he was poisoned. But how the poison got into his drink at the PR event while the police were interrogating him is still a complete mystery. The only thing he had to eat or drink was a soda, but Kurt Pressman sipped from the same cup and was unharmed.
The next day, Monk and Natalie pay a visit to Kurt Pressman's garage. Monk has gotten hold of a prototype of the high-tech vacuum, which has broken, and he wants the inventor Pressman to take a look at it. Pressman seems to have trouble getting his tool belt to fit and can't find several of his tools. However, he promises Monk that he can fix it and that it'll be ready later in the week.
Natalie and Monk pull away from the house in Natalie's car when Monk notices that they're being followed by a black SUV. Natalie speeds up, the SUV speeds up; Natalie turns a corner, the SUV turns a corner. Monk starts to panic. Then, when she turns down a deserted street they hear gunshots! They flee on foot through an industrial park. They hide in a dumpster. From outside they can hear voices. Soon the dumpster starts moving. When it comes to a stop, they cautiously lift the lid to peer out, and...
SURPRISE!! Monk finds himself in a warehouse filled with friends and coworkers. Natalie got him after all! The whole chase was orchestrated and the bullets were squibs. Everyone is laughing and having a good time, except for one of the guests, Kurt Pressman, who's surreptitiously dropping some square ice cubes into Monk's drink.
When Monk sees a partygoer adjust his belt, the solution to the case comes to him. He remembers Pressman's belt not fitting him during his pathetic attempt to fix his vacuum. The reason Pressman couldn't fix the vacuum was because he never built it in the first place. It was Bradley Foster's invention all along. Foster took his invention to Meckler, who then schemed with Pressman to steal the idea. Meckler killed Foster in the trash compacter, while Pressman pretended to be the inventor. They were supposed to share the profits, but Pressman got greedy and killed Meckler as well.
Right before Monk takes a sip from his drink, he realizes - his ice cubes are square, just like the ice that was in Meckler's cup when he died. Everyone else's ice cubes are round. Pressman has been putting his poison in the ice cubes! That's how Pressman was able to take a sip of Meckler's poisoned drink -- the ice cubes had yet to melt, so the poison wasn't released. Pressman is arrested, and later Monk thanks Natalie for the best birthday party he's ever had.