In a secret place at the end of the world, there's a gigasmic sunken treasure with sparkling jewels and gold doubloons. When the sunlight bounces off it and shines into the sky, it creates dazzling rainbows. In fact that treasure is the source of all the rainbows in the whole world. Betcha didn't know that. Since submarines Olly and Beth are ocean explorers, they set out to find the rainbow treasure, but a whole bunch of bad guys are looking for it too. Like Snarky Sharkskin and his slippery Seahorse Gang. When the treasure's good Sea Lion Guardians turn into evil Sea Lion Pirates, the rainbow is in deep danger. If they steal the treasure, then all the rainbows in the whole world will be lost forever. Oh no. How can Olly and Beth save the rainbow? There's only one way. "Dive, Olly, Dive!"