Join Cassie, Katy, Zia and Thunder the cat on an underwater adventure

On the hottest day of the year, the girls’ playdate is unexpectedly moved to Katy’s flat. But it doesn’t matter that Katy doesn’t have a garden because the normally shy Cassie has the perfect plan for their next adventure. Together they imagine a wonderful waterslide coming out of Katy’s bedroom window, winding down and down and shooting out straight into the sea. The slide takes them on a super-slippery adventure like no other, where they meet a host of sea creatures, including a dolphin who shows them why the ocean is in trouble. The creatures need their help, and the girls and Thunder must use all their brainpower to help put things right before they can go home.