A man dressed in Holmes-period clothes stabs hotel waiter Robert Perez in the back, carefully arranging the crime scene. When Nash and the SIU (Special Investigations Unit) intercede as the man tries to get away, he fires a derringer at Nash and Joe. Before Nash and Joe can capture the man, a "Dr. Watson" fires from behind covering the man's escape. "Dr. Watson" is taken into custody, but he escapes also.
The killer has struck three times in the past week, each time arranging the homicide scene in a tableau from a story featuring fictional detective Sherlock Holmes. A pair of glasses placed on the dead body in the latest killing is a tableau from The Adventure of the Golden Pince-Nez.
Tracing the antique guns used by "Dr. Watson" to an antique gun dealer, "Watson" is identified by his application for a handgun purchase as Sidney Dunlop. Nash and the SIU grab Dunlop. Dunlop reveals that the killer is Nigel Poole, a dangerous mental patient who killed his parents at age fifteen. Their game of Dungeons and Dragons has gotten out of hand. During the police interrogation, Poole sneaks into squad room and spies on Nash plotting strategy to capture Poole. Poole sees this as a challenge and focuses in on Nash.
Meanwhile, Joe's baby Lucia has been cholicky and keeping Joe and Inger awake all night. Nash suggests his former baby nurse Debbie. Nurse Debbie turns out to be very bossy and takes over the Dominguez household, including the den area. She smokes, and forces Joe to replace her favorite piece of boysenberry pie he has eaten at 3AM in the morning. A harassed Joe calls up Nash to go search for the pie - they locate a piece, but Joe must trade his jacket away because another customer has claimed the pie before Joe. Evan is worried about Department Competency Test, in which all members of the SIU must take and pass. Harvey agrees to tutor Evan on the test.
Poole kills the antique gun dealer (who happens to be his cousin). Poole leaves behind a series of clues for Nash, including one at Nash's desk in the squad room, and another at the homicide scene depicting The Adventure of the Final Problem, in which a struggling Holmes and Professor Moriarty tumble to their deaths over a cliff. Poole calls Nash from his home (fortunately Nick is away in San Diego) and drinks Nash's wine. Nash consults Poole's doctor for further insight into the killer. Dr. Gabrielle advises Nash that he might be the next victim.
Poole's next victim is Dunlop, and Poole leaves clues at the homicide scene indicating that Dr. Gabrielle is next. Nash and company intercede and save Dr. Gabrielle. Nash confronts Poole on the roof of Dr. Gabrielle's apartment. Poole lunges at Nash, throwing the both of them over the side. Poole falls to his death, but Nash hangs on to a light pole.
The entire squad passes their test, with Evan scoring the highest, but Harvey barely passing. Dr. Gabrielle asks Nash out to lunch in appreciation for saving her life, but Nash must accompany Joe to fire nurse Debbie. Before Joe can, nurse Debbie announces she is done, leaving behind a well baby Lucia.
For more, see commentary for Episode 35