While the world celebrates the spectacle of the rare comet, Cleo, Rikki and Bella know it’s destined to strike earth and set out to stop it. Bella locates an ancient tale from when the comet last passed about how a tower of light shot from the earth, touched the comet and saved them from cataclysm. On hearing that a young girl named Eva was responsible for the light, the girls realise the tentacle’s potential. Meanwhile, in an effort to recover more unique crystals, Sophie and Ryan damage the Moon Pool’s ability to create the magical energy the mermaids need. The girls combine their powers to create their own tower of light and risk everything in an attempt to direct it toward the comet. They succeed and prevent a collision, saving everybody and fulfilling their destiny as mermaids. Later, Rikki, Cleo and Bella graduate from school to enter a new phase of their lives, forever bound by the sea and their shared secret.