After Bullwinkle drinks a monster serum, he can't control when he turns into a big hairy monster, so he and Rocky go on the run from angry mobs and internet celebrities.

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.