On the night of the full moon, Zac and Erik swim off to Mako Island and unlock the merman chamber when Mimmi and Ondina go to rescue them Mimmi and Zac find out a startling secret that they're siblings and that's why they share their visions together. While Sirena and Evie go to the moon pool to help Evie get over the moon spell. Also Zac finds out he has been a merman his whole life, but his real mother put a protection spell on him and it was broken when he fell into the moon pool.