Marnie Blue is shocked when lots of plastic rubbish starts to appear in Mermaid Lagoon. It's causing all sorts of problems and even harming the underwater animals. Marnie and her friends decide enough is enough and they must have a big green clean-up. But just where is all the plastic coming from? With the help of the local Brinies group, a new dolphin pal and a human friend, the mermaids come up with a plan to rid the lagoon of plastic junk for good.