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