A tollbooth operator is killed at work. Across town, a woman is killed in a movie theatre. Those are just two more victims the police have added to the growing number of bizarre homicides that have sweeping the city in recent weeks. Immediately, Monk is suspicious of the details surrounding both murders and by the clues - or lack thereof - left at the various crime scenes.
With the city under a cloud of fear, the entire San Francisco police department has been transformed into a task force with one goal: apprehending the killer. Monk arrives and begins to review photos of the victims, and in typical Monk fashion, he deduces a connection between some of them which leads him to the home of the next victim. But he's too late - the eleventh victim, a man named Henry Smalls, has already been murdered!
Monk returns to the police station, analyzing the evidence before him. It just isn't adding up: the victims are eleven extremely different people with extremely different backgrounds. Yet they're all from the same county, and all registered voters... Finally, Monk figures it out - they all served together on a jury!
It doesn't take long for the police to locate the twelfth and only remaining member of that jury, Wallace Cassidy, and when a human fingertip turns up in his freezer, the police are certain they've got their killer. But Monk isn't convinced. He believes the answer lies with Charles and Lisa Babcock, the defendants from the original lawsuit.
It's at the Babcock's house that Monk pieces it all together: back when the case went to trial, the jurors visited Babcock's house to view the "scene of the accident." While wandering through the house, one of the jurors stumbled across something big, and then started sending Babcock threatening notes in the mail. It turns out that Babcock was being blackmailed by a member of the jury, but not about the lawsuit he was involved in: for the murder of his first wife.
Babcock had hidden his wife's body in his house, but someone on the jury had found her corpse, and now Babcock was paying the price for that discovery. Babcock originally paid off the blackmailer, but six years later, the blackmailer came back looking for more money, and that's when Babcock decided to take matters into his own hands - by killing off the jurors one by one.
A pretty difficult case to crack, and the burden of proof rests squarely on Monk's shoulders. But Monk notices something interesting from the eleventh victim's murder scene: a piece of the assailant's shirt was found clenched in Henry Smalls' hand. Monk recognizes the one-of-a-kind stitching on the shirt's button as the handiwork of Mrs. Ling, Monk's drycleaner. Monk then nails Babcock by getting Mrs. Ling to positively ID him as the owner of the shirt in question.
In the end, Charles Babcock likely would've gotten away with murder - if it wasn't for Adrian Monk's unique attention to detail.