Trama

Lewis find out that there is a way for a mermaid to lose her powers and attempts to lure Charlotte to the Moon Pool. The plan backfire and it looks like all the girls may lose their powers.

Max Hamilton tells Lewis that this full moon is different. When the planets align, the moon can take all a mermaids powers away for good if they are in the moon pool when it passes overhead. Frustrated with Charlotte, he decides to trick her to getting into the moon pool. Cleo discovers that Lewis and Charlotte have gone to the moon pool and she gets worried and decides to swim out to see what is going on. When all 3 girls arrive, Charlotte reveals that she knows about the special moon and attempts to get Emma, Cleo and Rikki into the water instead.—Anonymous

A rare full moon with the power to strip away the girls' powers permanently is approaching. The girls attempt to forewarn Charlotte, but Charlotte instead uses her powers against them out of spite. She releases her hold on them once Ash comes out; Emma attempts to make Ash forget what he saw, but not giving up, he visits her that night to try to get an explanation. Charlotte, still bitter, controls the water in Emma's house from the outside, and the girls cannot fight back unless they end up revealing their secret to Ash. When Cleo follows Lewis and Charlotte to Mako Island, Rikki and Emma join her as well. Cleo, Rikki and Emma have a power battle with Charlotte, in which Charlotte loses both the battle and her powers. The following morning, Lewis retrieves the locket from Charlotte, Emma shows her secret to Ash, and the season closes with the girls running into the ocean while their boyfriends look on.

On the night of the full moon, a rare planetary alignment presents a unique danger to the girls - the permanent removal of their powers. Their attempt to warn Charlotte, who has yet to experience a full moon, only renews their rivalry and sets up a confrontation that will change lives forever. Tired of all the secrets, Ash demands an explanation from Emma.

With the full moon close Cleo, Rikki and Emma must remain inside if they don't want to be exposed to a rather nasty shock. After trying to warn Charlotte the girls hide out with Lewis to escape the Moon's powers, but not for long when they soon realise nothing is going to stop Charlotte's rampage. The ultimate battle is won at the moon pool and someone's going to lose their tail forever. But who ?