Rodent Monster is a game about Beastie. A lab rodent that in an accident gets all the power's of the monsters in the world. So he can became a vampire, werewolf, the monster of the lagoon and Hyde.
Doctor Spector, Beastie's master has been working hard in some experiments to get the monster essence which is the thing that make a monster a monster.
The Evil Phantom hear this and kidnap Dr. Spector to apply his experiment for military use.
So it's up to Beastie to escape from the lab cross the woods and the swap to defeat Phantom and rescue his master.