The monsters are going to be submerged in the school pool. They go deep inside and without knowing how they end up teleported to the bottom of the sea, a place very far from the pool in which they were.
They do not end up in that place by chance, but end up in the Great Barrier Reef because that is the home of Lagoona, one of the girls. There he has a mission to fulfill, which he has postponed too long.
Lagoona is going to have to face the bully of his childhood, his nemesis. In this way, you will have to face your past and your deepest fears, something you do not like, but you know you have to do.
In addition, you will also have to face a huge beast from the depths of the sea, so the trip will not be simple for either Lagoona or the monsters that accompany her in her new adventure.