Nia is a young girl living on the west coast of Ireland. She has always loved the ocean and wishes that she could see a mermaid. Two young mermaids overhear her say this, and grant her wish, appearing to her one day as she explores the beach caves. The mermaids take Nia on a journey to experience their ocean world. Together the three friends see amazing sights during their underwater voyage, including a clever but mischievous octopus, and a turtle who is hundreds of years old.