Submarines Olly and Beth join their friend Bobsie, who is convinced he is the great-grandson of the Rainbow “Pie-Rats”, to search for the legendary secret treasure that reflects the beautiful rainbows in the sky. Their exciting treasure hunt is filled with wonder, comedy, thrills and adventure. But, they are followed by the unscrupulous Snarky Sharkskin and his slippery seahorse gang. As resourceful as ever, Olly and Beth manage to overcome all the problems they encounter, survive the storms at sea, make a heroic rescue, and find the glittering rainbow treasure. However, in the end, the treasure hunt leads them to understand that the best treasure of all is family and friendship.