Sirena and David were on the beach, and Sirena almost spoils the secret, so she dives into the water. Meanwhile, Joe got a new boat, and Sirena gets tangled in a net trying to swim to Mako. Joe finds her, and keeps her a secret. David knows he's hiding something, and forces Joe to show him what it is. Will David be able to get Sirena out of this mess, and keep the mermaid secret?

When Emma returns home, the girls decide to spend the night at Mako. When a strange comet made of moonstone comes crashing in, the girls are on the case. Rikki discovers that whenever they're around moonstone, their powers get stronger. Why would their powers need to be advanced? Would something be threatening the moon pool, and would they have to protect it before it's too late?

The girls are curious as to how Rita's laptop works, so Zac teaches them it's functions. A simple internet search on mermaids leads to much more.

Even though Harry is gone, Rita will always be thinking of him. This is Rita and Harry's story before the tragedy.

Elizabeth and June are two best friends who live in California. One day, they go off of school grounds, and to a strange beach. A note in a bottle, and two gorgeous necklaces give these girls the surprise of their life when the full moon rises over the waves. Can these girls handle suddenly becoming mermaids, keeping the secret, avoiding water, and being freshman in high school?