When an armored car driver is shot to death outside a grocery store, Monk is called in to investigate. Witnesses saw the shooter, who used the driver's own gun for the shooting. Monk looks for clues, but before he can get too deep into the case, he gets a phone call from his brother, Ambrose. Apparently, their long-lost father called and is coming to Ambrose's house that night for a visit. Monk is skeptical, but he decides to go see Ambrose anyway.
Monk, Natalie, and Julie arrive at Ambrose's house. It's Halloween, so the streets are alive with trick-or-treaters and spooky decorations. Even Ambrose is ready for the holiday: he's counted out exactly how many candy bars he needs for the trick-or-treaters in his neighborhood. When one trick-or-treater dressed as Frankenstein wants more than his share of Ambrose's candy, a struggle ensues, and Frankenstein hits Ambrose over the head with a plastic pumpkin.
Later, when Julie goes out trick-or-treating with some friends, Frankenstein strikes again, scaring the kids and stealing some of their candy. Monk arrives on the scene, and realizes that Frankenstein only took candy from kids who had been to Ambrose's house. Monk deduces that Frankenstein must be after something in Ambrose's candy.
Later, while trick-or-treating with Julie, Monk comes across a dead pigeon. Monk recognizes the pigeon: he saw it at the crime scene eating the murder victim's candy bar. Monk has Stottlemeyer autopsy the pigeon, which tests positive for poison. Then Stottlemeyer tests the dead armored car driver, and he too tests positive for the same poison - a poison that a man named Paul Gilstrap was caught returning to a lab that very day. Eureka! Monk can explain everything, but first, he needs to tell Ambrose not to eat any of his candy.
But it's too late - Ambrose has already eaten a poisoned Neptune bar, and is rushed to the hospital in an ambulance as Monk tells him what happened: while Monk and Julie were trick-or-treating, they met Paul Gilstrap's wife, who told them she eats a Neptune bar every night. Paul Gilstrap must have poisoned his wife's candy bar, and then planted some more bars at the grocery store to make his wife's death look like the work of a random killer. What he didn't count on was getting caught trying to return the poison, so Gilstrap needed to abort his plan. He got to his wife's candy bar in time, but by the time he made it to the grocery store, two of the poisoned Neptune bars had already been sold. One went to the armored car driver, who had already taken a bite. By the time Gilstrap reached him, he was already overcome by the poison. Thinking fast, Gilstrap grabbed the driver's gun and shot him repeatedly, thinking no one would look for poison in a man who was shot multiple times. The other Neptune bar was delivered to Ambrose, and Gilstrap ' disguised as Frankenstein ' was trying to get it back all night.
Unfortunately, Monk's solution came too late for Ambrose' or did it? Monk notices another unopened Neptune bar in an evidence bag, and it turns out the Neptune bar Ambrose ate wasn't poisoned after all!
With Gilstrap in custody and Ambrose not in danger, everyone returns to Ambrose's house, where Monk and Ambrose find a note from their father. He did indeed stop by to visit, but they missed him! Perhaps someday the Monk brothers will reunite with their dad, but until then, they have each other.