Merman Mariano hails from the lost city of Atlantis, which is not too far from a sprawling coral reef. The coral reef is home to amazing creatures like the blue parrotfish, angelfish, and even the nurse shark. One of the main sources of food for the Atlanteans is the algae that grows on coral. Unfortunately, the coral supply is depleting and it's up to Merman Mariano to figure out what's going on and how to stop it! While on his coral quest, Mariano meets Mermaid Tasha. With her help, Mariano comes to land to teach the humans what they can do to protect the reefs by being responsible divers and fishermen, using reef-save sunscreen, and recycling.