The Chipmunks work in an amusement park attraction. After Alvin drives a crazy tour group, they miss their next performance and are locked in the park after closing time. Little do they know that the real Dr. Frankenstein has been hired in a new attraction called, "Frankenstein's Castle"; figuring that the castle isn't scary enough, the mad scientist recreates the real Monster.

As Alvin struggles with nightmares involving werewolves, the school play of Jekyll & Hyde is in jeopardy when the lead, Alvin, is suspended. In an attempt to boost his confidence, the principal suggests Theodore, who's suddenly going through some canine changes.