Cordelia, her sisters Marilla and Isla are loyal sisters who lived by the sea shore with their Grandfather Derek on a Hawaiian Island called Maui. They like swimming, exploring sea caves behind waterfalls, playing with sea animals and each other, and what they like the most is surfing. But one day they met a merman who got tangled in a fishing net in their favorite sea cave, along with a sparkly pink dolphin and sea pup.
They soonly told that they're half mermaid and half human, and it's up to them to save their underwater home from Marlow.