Life is trouble-free on Mako Island until the day that Ondina meets Weilan, a new mermaid from the Eastern pod. They immediately clash and Mimmi has to act as peacemaker. But a far greater threat to the peace of Mako looms, in the form of a fearsome Chinese water dragon that once destroyed the Eastern pod and killed Zac and Mimmis mother, Nerissa.
Weilan and Zac team up to fight the dragon with Eastern magic, but their plan backfires and the dragon takes Evies tail. Ondina blames Weilan and they are forced to work through their differences, realising how much they need each other if the dragon is to be defeated.
A shocking twist leads Mimmi to the conclusion that her mother was not killed by the dragon after all. Zac and the mermaids will discover even more amazing secrets until a final confrontation with the dragon reveals all.