The colours are disappearing from Cockleshell Cove! The bright coral reef is grey and dull, and even the clownfish have lost their colour. The MerTales crew are worried their tails might be next!

Sandy sets out to investigate with the help of her friends, Pearl, Shell and Coral. F.I.N.S. have saved Cockleshell Cove from reef dragons, giant turtles and catfish burglars, but the Mysterious Rainbow Thief moves faster than anything they have ever seen.

Can the mer-crew catch the thief and bring back Cockleshell Cove's beautiful rainbow?