Zacisateenageboywhodecidestocam onMakoIsland,unawarethatthreemermaids,Sirena,NixieandLyla,whoareguardiansoftheisland,arewatchinghim.Thatnight,whenthefullmoonrises,theboycomesintocontactwiththemagicwateroftheMoonPool.Thefollowingmorning,Zacwakesu discoveringhehasbecomeamermanwithafish-liketailandtheabilitytocontrolandmani ulatewater.Hisnew-foundmermanabilitieswillcause roblemsforthemermaids.Thethreemermaids,curiousaboutlivingonland,aretransformedintohumanswhileinthemoon oolduringafullmoon,nowhavingtheabilitytolivelikehumans