Lanny is a small planarian that lives inside an aquarium of a school. 

He is friends with a seahorse, Danny. One day, the aquarium managers discovers that there is a "planarian infestation" inside the aquarium, and decides to clean it, transferring all animals and plants to a new aquarium.
Danny loves Lanny so much that it doesn't want to miss Lanny in the new aquarium, so it gives it the idea to give pieces of himself to everyone so it can be "reglued" after the process.
Then starts Lanny's adventure through the old aquarium, trying to give everyone a small piece of himself.