Lewis discovers someone called Max Hamilton did research fifty years ago on the strange properties of Mako Island. Lewis tracks the old man down and finds that he knows a lot more about mermaids than Lewis first thought.
Under the influence of a Full Moon, the girls' powers are hugely enhanced and they wreak havoc on Lewis in his boat. The next day the girls realise they still need Lewis.
Rikki comes to stay at Emma's house and stirs up the family unit. In trying to compete with Rikki, Emma tries to rebel and throws a party. Nate uses Lewis to make a play for Cleo.
Emma meets Ash - a handsome horse riding instructor. A horse falls ill and Emma must use her powers to fix it. Lewis accidentally invites both Charlotte and Cleo to an awards dinner.
Zane has an argument with a man in a trailer park but little does he know it's Rikki's dad. Rikki has to overcome her shame about her background and introduce her dad to Zane.
Charlotte follows Lewis to Max Hamilton. Max tells all to Charlotte when he discovers she's the granddaughter of his one true love. Charlotte and Lewis almost break up until she goes to the moonpool, becoming a mermaid herself!
Lewis finds an old book on mermaids and tests out a potion recipe on the girls. Cleo tries to get Don and her Mum together by using her powers to grant him a wish. Realising it's not what he wants, she finally accepts the situation.
Cleo is failing at school and is assigned a tutor - Charlotte. Cleo wants Lewis back but her attitude to Charlotte only drives him further away. Cleo discovers Lewis and Charlotte kissing.
Charlotte takes over and organises a surprise party for Lewis, annoying Rikki. At the party, Charlotte is jealous of Cleo and traps her and Emma in a pool room. Rikki comes back just in time. From now on, Charlotte is on her own.
Emma uses her new powers to help Elliott win at soccer. Lewis decides to test the girls' increased powers. Cleo has had enough of Lewis' smothering and dumps him. Lewis meets Charlotte and they bond.
Emma thinks she's going to get a promotion at the juice bar but Ash is given the job. Emma and Ash clash over the job, but we discover it's over a lot more. Meanwhile, Lewis stands up to Charlotte over spending time with Cleo.
Zane's back in town and still in love with Rikki. He works out she's still a mermaid and tries to win her back. Cleo becomes suspicious of Lewis spending time with Charlotte.
The girls have to deal with a fourth mermaid who turns out to be a loose cannon; Charlotte almost turns into a mermaid at a volleyball competition. Charlotte discovers she has all three mermaid superpowers.
Zane hears of a treasure under the ocean and enlists the girls to find it. Rikki is injured in the search. Zane gives Rikki the reward. Charlotte contrives to keep Lewis from the girls.
Emma starts acting weird after pricking her finger on a barb of coral. Cleo's fish Hector exhibits the same behaviour. Emma becomes more and more like a fish and Lewis has to find a way to cure her.
Cleo and Rikki push Ash into asking Emma out again – little did they know he'd turn up at Emma's place in time for a full moon! Rikki and Cleo are both moonstruck and Ash looks set to discover Emma's secret.
Cleo is bullied by Charlotte and runs away to the ocean. With Cleo missing, everyone becomes more and more worried. Lewis discovers what Charlotte has been up to and breaks up with her. He finds Cleo and they kiss.
The full moon rises, and with the planets lining up, it can remove mermaid powers for good. Knowing this, Lewis lures Charlotte to the moon pool but it backfires and Emma, Cleo and Rikki all look set to lose their powers.
Everyone is shocked when Cleo is fired at the Ice Cream Stand, and even more so when Lewis takes her place! Rikki and Emma strike back at him but end up going too far.