On the hottest day of the year, the friends' latest playdate is at Katy's flat and 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 ocean. 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 girls and Thunder must use all their brainpower to help put things right...
