Marin and Urin are sea folk, people who live in a peaceful underwater world surrounded by bright fish and colorful coral. But one day, a beautiful ring falls from the "sky" - that dangerous place above the water - and the two girls decide to return the lovely trinket to its owner. While the surface world holds amazing wonders for the pair, it also hides a great darkness... a darkness that Urin accidentally unleashes when she opens a stone box at a local shrine. Now it's up to Marin, Urin, and their new friend Kanon to set things right again before that darkness swallows the entire world!
Episode 1
Two mermaid sisters, Marin and Urin, discover a ring that has fallen in the ocean and attempt to return it to the owner. However, when they find Kanon she refuses to accept the ring and throws it into the forest. Urin gets lost in the forest during a fireworks display and finds a sealed tomb.
Episode 2
With Sedna's release, darkness washes over the island. Matsumoto, the elder sea turtle, awakens and explains to Marin that she is the Priestess of the Sea. In disbelief, Kanon runs off and is attacked by a creature of darkness.
Episode 3
Kanon is shocked to discover that Marin, Urin, and Matsumoto have made themselves at home in her house. Marin poses as a fortuneteller to distract Suzuki from finding Kanon, who is hiding, and learns more about Kanon and her mysterious evil aura.
Episode 4
Matsumoto gives Kanon a treasure map for Kagero Island, hoping that if she finds the treasure it will improve the purity of her heart. After Kanon and Marin leave, Miyako explains to Urin of a legend about a pirate's cursed treasure within the cave.
Episode 5
Amamiko Island is abuzz with activity as tourists flock to see the solar eclipse and Matsumoto fears the darkness such an event could invite. Meanwhile, the growing rift between Urin and Marin reaches a breaking point.
Episode 6
The solar eclipse blankets the island in night as Marin and Kanon search for Urin. Countless mysterious butterflies and a strange woman appear from the ocean.
Episode 7
In the wake of Urin's departure Matsumoto's daughter, Ichikawa, is discovered in Kanon's house. A downcast and shaken Marin decides to return to the ocean to search for her sister.
Episode 8
While heading back to the surface, Warin explains the nature Marin and Urin's relationship to Kanon. A concerned Kojima searches for Kanon, leading to a confrontation between her and Ooshima.
Episode 9
Marin returns to Amamiko Island in order to regain her ability to transform. Suzuki and Kojima tell Marin about how they met Kanon, and along the way comes to understand her own feelings, as well.
Episode 10
No overview available.
Episode 11
No overview available.
Episode 12
No overview available.
