Atlantis is said to have prospered with a high degree of civilization, holding a paranormal power beyond human knowledge. But because of its unrelentingness, the island was sunk deep into the sea by Zeus, the omniscient and omnipotent god.

Despite the collapse of the civilization, Atlantis' descendant "Nereus" survived, evolved his power into a form suitable to live underwater, and built the Nereus Kingdom. The Nereids royal family has already become an incarnation of spirits, and the 50 daughters collectively known as "Nereids" have the power of fairies. However, the kingdom is on the verge of extinction due to the hands of Poseidon, the sea god who fears its mighty military power. King Nereids makes Nereids time-leap to each era with special abilities in exchange for the loss of many of their memories, but four immature sisters are all transferred to the same era.

This is the story of the Aqua girls who enjoy their days without worrying about anything else!