Welcome to Mermaids Rock! The entrance to the mermaid realm in the deep, blue ocean. Kai is really excited that Marina is coming to stay while her dad goes on a research trip to the Arctic - they're going to have so much fun! But when they don't hear from him for a few days they start to worry. Travelling through the whirlpool to see if they can find him, the friends are amazed by the icy scene and enchanted by the walruses they meet. But with no sign of Marina's dad and time running out, can the team work together to save the day?