When a young man tries to trick-or-treat dressed in bloody clothing and carrying a knife, Monk knocks him out and tells Natalie he's not in costume – he really has just committed a murder.

The next day, Stottlemeyer sends Monk and Natalie to Trouble, California, a small town known for an unsolved train robbery that happened there in 1962, to investigate the murder of a museum security guard who was a retired San Francisco police officer.

The two then investigate and discover that in the 1840s, Trouble was also home to one of Monk's distant relatives, an assayer who possessed skills invaluable to the small town, and exhibited many of the same obsessive compulsive traits as Adrian.

Kelton is reprimanded for trying to apprehend Gator without backup, and Monk and Natalie instead try to question Clifford Adams, the engineer of the train that was robbed.

The next day, Natalie berates Monk after he admits to using her to lure Kelton to the museum in order to expose his involvement, and forces him to apologize by throwing French fries at him.

[4] In the story, Artemis Monk, an assayer in the California gold rush town of Trouble in the 1840s, goes from determining the value of rocks to solving a murder.