Monk's biggest fan, Marci Maven, who's as nutty and intense as ever, is at home redesigning her Monk web site. Meanwhile, across the street, John Ringel returns home from a jog and discovers his wife mauled to death by a wild animal. Suspecting Marci's vicious dog, Otto, Ringel calls the police, who arrive at Marci's house to question her. But Marci insists it couldn't have been Otto. Otto is dead, and she shows them the dog's grave to prove it.
Marci wants Monk's help, but he refuses to talk to her, so Marci takes advantage of a police "bachelor auction" in which Monk is reluctantly participating. Marci attends the auction and gets her man by outbidding Natalie for Monk's services. With the money going to charity, Monk has no choice but to honor the results.
Monk and Natalie are unsettled to discover just how obsessed Marci is when they arrive at her house the next day. Eager for a distraction, Monk sets about investigating Otto's grave. Marci explains that her dog had been dead for three days when John Ringel's wife was killed. Unfortunately for Marci, the police exhumed the dog's body and found that the dog's teeth matched the bite marks on the victim's body perfectly. Stottlemeyer suggests that Marci had put Otto down after the mauling to avoid manslaughter charges, as Marci had been warned twice about her aggressive dog.
But Marci insists that someone framed Otto, and presses Monk and Natalie to investigate. They sneak onto John Ringel's property to look around, and Monk finds clues that call into question the police's theory of what happened. Just then, Ringel comes home and discovers them snooping! Ringel pushes them out, but as he does, Monk spots one final clue that leaves him convinced that Otto might not have been responsible for Ringel's wife's death.
Later, Marci remembers that Otto disappeared for an entire day just two weeks earlier, and when he returned, Otto was covered in woodchips. Monk examines Otto's hairbrush and finds teak and mahogany shavings. Conclusion: Otto had been to a lumberyard.
Monk, Natalie and Marci begin checking out lumberyards, and Marci impresses Monk with her crisp officiousness. Marci offers to work as Monk's assistant for free, and Monk, always looking for a bargain, briefly considers Marci's offer. Natalie, offended, decides to leave and give Monk a chance to see what working with Marci would really be like.
Monk and Marci spend hours visiting lumberyards until finally they enter one owned by John Ringel. The ditzy Marci forgot to mention that Ringel owned a lumberyard! Ringel shows up and immediately calls his lawyer. While Ringel talks on the phone, Monk takes the opportunity to look around. He notices some chew marks and dog hair on a chair in the corner, and a can of plaster on a shelf.
Monk has an epiphany! Here's what happened: Ringel had been planning to murder his wife for weeks. He realized he could blame it on Otto because of the dog's violent reputation. On the day Otto disappeared, Ringel kidnapped the dog and brought it to the lumberyard, where he made a dental mold of the dog's teeth. Ringel then fashioned a murder machine by attaching a metal casting of the dog's teeth to a pair of garden shears, so Ringel could kill his wife with the "machine" and it would appear that the dog had done it. Unfortunately for Ringel, by the time he killed his wife a week or two later, Otto had died, and Ringel had framed a dead dog!
But Ringel has been listening to the entire summation, and now he's got a gun pointed at Monk and Marci. Monk throws some nails at Ringel's face, and Monk and Marci take off into the lumberyard's warehouse. After a chase, Ringel has Monk and Marci cornered. A terrified Marci surrenders, and Ringel grabs, drawing Monk out of hiding. Fortunately for Monk, Stottlemeyer has pieced together the case on his own, and he arrives, along with Natalie and Disher, just in time to save the day and take Ringel into custody.
It's one of Monk's most bizarre cases, but he solved it just the same, and this time, there's an ancillary benefit to the whole adventure. Marci is so unsettled by her private-eye experience, that she decides to move on from Monk and turn her obsessive attention to F. Murray Abraham.