ZacisateenageboywhodecidestocamponMakoIsland,unawarethatthreemermaids,Sirena,NixieandLyla,whoareguardiansoftheisland,arewatchinghim.Thatnight,whenthefullmoonrises,theboycomesintocontactwiththemagicwateroftheMoonPool.Thefollowingmorning,Zacwakesupdiscoveringhehasbecomeamermanwithafish-liketailandtheabilitytocontrolandmanipulatewater.Hisnew-foundmermanabilitieswillcauseproblemsforthemermaids.　　Thethreemermaids,curiousaboutlivingonland,aretransformedintohumanswhileinthemoonpoolduringafullmoon,nowhavingtheabilitytolivelikehumans.