Bullwinkle and Rocky fly to a mysterious monster-inhabited island to meet the missing mad scientist that created the serum that turned Bullwinkle into a monster, only to learn of the biggest, meanest monster on the island, Humongalon.

When Fearless Leader uses the monster Humongalon to force everyone to surrender the world to him, Rocky and Bullwinkle try to stop them in the most massive monster battle ever.

When Rocky and Bullwinkle damage the largest birdbath in the world, they set out on a musical adventure. And Fearless Leader attempts to become a musical superstar so he can amass a legion of devoted super fans who will obey his every command.