Emma, Rikki and Cleo think they fooled everyone who discovered their secret into thinking they lost their powers. Only Lewis knows they're still mermaids. Life is back to normal for the girls, they go swimming on the underwater reefs, shop for the latest clothes - and avoid getting splashed with water. After all, no one wants to grow a mermaid tail in public! Along the way the girls have a lot to deal with: from magical mermaid myths about wish giving and love potions to more everyday problems like new girl Charlotte's arrival on the scene. Charlotte's ambitious, determined and anxious to discover why Lewis spends all his time with the girls. After learning her grandmother's secret, Charlotte makes a life-changing decision that will affect Emma, Rikki and Cleo in more ways than one. Can they handle a fourth mermaid?