It's a quiet afternoon in the San Francisco suburbs. Amanda Babbage chats on the phone as she retrieves her mail, confiding to her friend a recent series of difficulties between she and her brothers. Little does Amanda know that the package she's just carried into the house is in fact a bomb. This oversight is made clear a few moments later when, as she attempts to open the package, it explodes, killing her instantly.
Though Monk is in the middle of a personal crisis - his psychiatrist, Dr. Kroger, has left town for vacation, leaving Monk to deal with his obsessive compulsions on his own - Captain Stottlemeyer nevertheless brings him in to consult on the bombing. The investigation is being headed up by the ATF, and Stottlemeyer needs all the help he can get to keep his hand in the case. Monk quickly makes a few pertinent discoveries: first, he notices that the package was tied with an unusual and distinctive knot, and secondly, that although the package was indeed meant for Amanda Babbage, it was labeled with her former address. Whoever sent the package was unaware that she had moved just two months before.
As the officers continue to investigate the crime scene, a ruckus in the driveway brings everyone outside. There they find Ricky Babbage, Amanda's spoiled brother. It turns out that all three Babbage children have been embroiled in a legal dispute over the validity of their late father's will. Each sibling was suing the other over the family fortune, and that fact is enough to make ATF Agent Grooms suspect Ricky had something to do with the bombing. But Monk is quick to discredit this theory, stating that not only did Ricky know his sister had moved, but he tied his surfboard on top of his truck with a careless and sloppy knot. Neither of these details coincide with Monk's profile of the mail bomber.
With Ricky eliminated as a suspect, Monk decides the next likely candidate would be the third Babbage sibling, Brian. But Disher and Stottlemeyer are quick to dismiss the idea, as he's been in a coma for the last four months. Stottlemeyer explains that while he and Disher were apprehending a criminal, Brian Babbage sideswiped their car with his Mercedes, and goaded them into giving chase. But before they were able to pursue him, Brian's car was pummeled by an oncoming truck and sent crashing into a parked car. Babbage has been in a coma ever since.
Not convinced by this seemingly airtight alibi, Monk goes to visit Brian in the hospital, and later makes a trip to his home. At both locations, he finds clues that make it crystal clear who the culprit is. Firs, Monk finds a pair of Brian's tennis shoes - tied with the same distinctive and unusual knot as the package in Amanda's apartment. Second, Monk finds a number of ketchup bottles glued to the ceiling in Brian's bedroom closet, along with a handful of bottles that have fallen to the ground. That's enough for Monk to declare whodunit.
Monk deduces that when Brian sideswiped Stottlemeyer and Disher, his goal was to just be arrested, not end up in a coma. He figured his crime would be enough to land him in jail for a few months - just enough time to carry out his plot. Before instigating the police chase, Brian had adhered three packages to the insides of mailboxes around San Francisco - each addressed to one of the Babbage children, and each held to the top of the mailbox with glue. As the glue dissolved over a couple of weeks, the mail bomb would eventually fall in with the rest of the mail below it, and then be carried away. As he had planned to be in jail when each package was delivered (including one he addressed to himself), Brian surmised there was no way he could be accused of the murders - and, with both his brother and sister dead, he would then become the lone benefactor of his wealthy father's estate. However, when Brian Babbage finally awakens from his coma, he is shocked to find himself under arrest, and his fool-proof plan defused by the explosive genius of Adrian Monk.