The Chipmunks are performing at a theme park called Majestic Movie Studios (a spoof of Universal Studios Hollywood). While taking a break from their concert, the Chipmunks get lost, and eventually get locked inside the park. They find their way to the "Frankenstein's Castle" attraction, where a real Dr. Frankenstein is working on his monster. The monster is brought to life, and the doctor sends it in pursuit of the Chipmunks. In their escape, the monster retrieves Theodore's dropped teddy bear.
The monster follows the Chipmunks home and returns the bear to Theodore, who quickly befriends him. The Chipmunks learn that the monster (whom Theodore has nicknamed "Frankie") is truly good-hearted. Dave goes to the park to book a concert that night to celebrate the premiere of an anticipated film. Dr. Frankenstein tracks Frankie to the Chipmunks' home, and, angered at monster's benevolence, kidnaps Alvin. Simon, Theodore, and Frankie hurry back to the park to rescue Alvin.
Dr. Frankenstein force feeds Alvin a potion and induces a powerful electrical shock. Alvin is released by Frankie, and after Simon swipes the doctor's potions book, the four of them escape back into the park. Shortly after, the process Alvin underwent takes effect, transforming Alvin into a zany cartoon monster. Alvin escapes to the premiere, causing chaos and havoc in his path. Using the potions book, Simon and Theodore mix an antidote using various food items from a buffet, and feed it to Alvin during his rampage. Alvin returns to normal, and the Chipmunks go to perform their concert.
Before the concert begins, Dr. Frankenstein attempts to transform Alvin back to his monster self, but is thwarted by Frankie. Theodore introduces Frankie to the public, promising that Frankie will bring no harm if treated kindly. As for the doctor, he remains forever stuck in the Sammy Squirrel Costume and doomed to sing the song he despises so much.