There isn't a style or pop culture trend in the monster world that Cleo De Nile isn't on top of. But on a visit to the human world, Cleo discovers a tradition that completely escaped her: Monster Con, where humans gather to dress up as their favorite monsters.

Convinced it's what Monster High has been missing--and determined to prove to her family that she can be a leader--Cleo decides to put on the school's first ever Human Con. She even finds the perfect accessory for the winner of Best Dressed: a dazzling crown from the catacombs. But when Cleo tries the crown on, strange things start to happen.

As the Con draws nearer, Cleo's behavior becomes increasingly bizarre. Thank the gods for goulfriends! Clawdeen, Draculaura, Frankie, and Cleo band together to solve the mystery of the crown's origins so they can break the hold it has on Cleo . . . before she becomes a shadow of her mummy-rific self!