Siblings Maya and Joey are forced to embark on a perilous journey when they are shipwrecked off the coast of Maradonia. After encountering fierce warrior mermaids, the siblings find out that their only way home is through a teleportation portal . . . which doesn't work!

If they want to get home, the siblings realize they must join the mermaids and find a way to revitalize the portal before it falls into the wrong hands. Just when all seems to go smoothly, a United States military expedition arrives on the shores of Maradonia and begins their own search for the portal. To make matters even worse, the Dark Realm declares war on the mermaids. Fighting one enemy is bad enough, but contending with two invasive opponents may prove too much for the mermaids. In Maradonia and the Guardians of the Portal, Maya and Joey are in for more trouble than they ever imagined: the ultimate battle for control of the universe.