A disagreement between Coral, Filly, and Manta, three young mermaids who work together to build and safeguard the reef that is their home, must quickly be set right.

When the Octonauts, a team of eight animals who explore the ocean, discover a bleached and abandoned coral reef, they learn about the important relationships among animals, plants, and their habitats.

With help from their cousin Harry, eight-year-old twins Ella and Ethan discover who is illegally taking coral from the reefs near their aunt and uncle's beach house on the shore of Australia's Coral Sea.

"Ayla lives on a beautiful tropical island surrounded by a coral reef. Her mom is a marine biologist, and every day, the two go exploring together. One day, Ayla notices that many of the fish have disappeared, and the once-vibrant corals have turned pale. She and her mom set out to save the corals--but is it too late?"--