"Curse of the Necklace" is the Halloween episode of the Martin Mystery.
Plot[]
In Charlottetown, Prince Edward Island a last minute shopper looking for a Halloween costume in Paddy O'Brien's costume shop finds a necklace that Paddy doesn't remember. When she tries it on things get eerie and Paddy freaks out. At Torrington Academy Martin is looking forward to the Halloween Party. So is but he can't find a costume in his size so Martin improvises a ghost costume. Diana shows up minus a costume saying Halloween is too commercial then she and Java both crack up when they see Martin's baby costume. Summoned to the Center via a portal in Java's laundry they meet Billy (who's dressed as Martin) and after a run in with MOM's pet lizard monster Hilda (MOM's idea of a Halloween prank) they're sent to Charlottetown to investigate why Paddy nearly died of fear. His costume shop is now in a dead zone (surrounded by a circle of dead plants and colder than the rest of the neighborhood), has been wrecked and there's puddles of black goo on the floor which turn out to be a tasty blackberry puree. Heading for the hospital to interview Paddy they find the place deserted except for him and he's in shock. Despite this Diana manages to learn from him that everything went crazy when a customer put on a mysterious necklace with an odd design which he begins to draw until he's interrupted by the arrival of a giant raven whose wings beat like a hummingbird's that Martin identifies as a Lich. Java holds off the Lich while the trio retreat to the elevator. As they do it divides into multiple smaller versions but once they're in the elevator it reemerges and returns to Paddy's room. They return to find him missing but his sketch finished which they to Billy. Meanwhile Paddy (now wearing a leprechaun costume) is with his former customer helping her make chocolate treats with blackberry filling that have the necklace symbol stamped onto them. Then a trio of trick or treaters dressed as ghouls and chaperoned by a in a clown costume arrive and are the first to taste the treats.
Meanwhile Martin, Diana & Java wander the town looking for clues and find a house surrounded by a dead zone then see the trick or treaters now transformed into real ghouls attacking other kids while their chaperone yuks it up like a real clown. Martin tries one of the chocolates dropped by a ghoul-kid and begins acting like a baby and Diana takes charge and removes his U-watch to call Billy for help. Billy arrives and tells them the necklace symbol is the mark of Carlin, the queen of the netherworld. She's always been hungry to expand into the real world and the barriers between the worlds are thinnest on Halloween. She slipped the necklace over to possess a host body and build herself an army by turning people into the monsters they're dressed as through the enchanted chocolates. With Martin out of action Diana convinces Billy to take his place in the team and the trio head to the house in the dead zone pretending to be trick or treaters. When Carlin's host offers them chocolates they try to jump her only to discover she can repel them with ease and orders Paddy (now a Leprechaun) to destroy them but they manage to beat him and Billy steals the necklace. Unfortunately while Carlin's host returns to normal none of the other effects vanish since Carlin hasn't been banished back to the netherworld. Java offers to destroy the necklace but Billy fears that'll trap Carlin on Earth and she claimed that on dawn on November 1st her spells will become permanent. Then Baby Martin grabs the shiny necklace and becomes Carlin's new host. Neither Billy nor Java are willing to fight Martin so it's up to Diana, But before she can use the U-Watch against him Martin becomes the Lich Raven and grabs her with his talons. Diana throws the U-watch to Billy and he summons the X-staff and uses its net to trap the raven and save Diana. Then he proposes to feed the chocolates to the possessed Martin turning Carlin's magic against her creating spiritual feedback. It works and Carlin is forced back into her own realm and Martin and everyone else returns to normal. MOM arrives to pick up the necklace for safekeeping in a box with other enchanted and invite the trio back to Center's Halloween party but Martin and Java decide they'd rather try trick or treating and Diana chooses to stick with them since she's decided Halloween can be fun after all.