A thousand years ago a demigod named Maui stole the heart of the island goddess Te Fiti. Ever since then a darkness has been spreading across the sea destroying life on the islands.
Moana is the daughter of her island's chief and desperately wants to go out past the reef to explore the ocean but leaving the bay is forbidden. When the darkness starts spreading to her island her grandmother shows her seafaring boats from when the islanders used to use to travel the ocean and explains to her that the only way to stop the darkness is to capture Maui and force him to return the heart to the goddess. She also shows her the heart which the sea tried to give to an infant Moana many years earlier.
Moana takes a boat and the heart and finds Maui who is convinced that he's the savior of man and tries to steal her boat. With the help of the sea who is trying to help her, she convinces Maui to at least try to return the heart after a quick side trip to retrieve his magical fish hook, the source of all his powers.
They regain the fish hook and go to the island where Te Fiti is only to find it blocked by a giant monster. Maui's fish hook his damaged and his powers are almost lost so he flees and Moana begins to give up but is inspired to go and return the heart herself. Maui returns and destroys his hook protecting Moana as she sails past the monster to where Te Fiti's island is. Upon getting there she discovers that the island is missing and realizes Te Fiti is the monster. She sings to her and returns her heart, causing her to transform back into the goddess. Te Fiti restores Moana's boat and Maui's fish hook. Moana sails home to her family and teaches her people how to sail so they can resume island hopping.