The story centers on Marin and Urin, two “pure” sisters who live alongside the fish in the sea, but yearn to be in the world beyond the water and above ground. One day, a beautiful ring falls in the middle of the sea, and Marin and Urin retrieve it. The two decide to leave their waterbound world for the first time to deliver the ring. After an arduous journey, they come across an isolated island where a high school girl named Kanon lives. Accidentally, Urin unleashes a power sealed long time ago in the island, and the only ones who can defeat it are Marin and Kanon, as priestesses of the sea and the sky.
Characters
;Marin A sea dweller who lives with Urin, Sam, and Warin. While people of the sea are born from shells and thus lack parents or direct family, she considers Urin her sister, whom she showers with affection. When Kanon's ring is thrown into the sea, she is the one who finds it and decides to search for its owner. Later, she becomes the priestess of sea and starts fighting Sedna along with Kanon. ;Kanon Miyamori A high school senior who lives at the island. She is the owner of the ring that Marin found, which she angrily discarded when she decided to break up. After Urin unleashes the power of Sedna, she becomes the priestess of sky. ;Urin A sea dweller whom Marin sees as her little sister, and who sees Marin as her big sister. She is the one who accidentally unleashes Sedna's power. In the middle of the series, she becomes Sedna's vessel. ;Warin A sea dweller, friend of Marin and Urin, and frequent companion of Sam. ;Sam Another sea dweller who is a friend of Marin and Urin, and frequent companion of Warin. ;Matsumoto An old turtle whose form was a statue in the island's forest until it came to life when Sedna was unsealed. Speaking human language, he advises Marin and Kanon regarding their roles as priestesses, comes to live in Kanon's house, and can generate the fireball needed for one of Kanon's spells. ;Kojima Kanon's ex-boyfriend. He and Kanon have some feelings toward each other, and Kojima once gave Kanon a ring, but at the start of the series, Kanon ended their relationship over due to tensions regarding how it was evolving. ;Oshima A schoolmate of Kanon and Kojima, she is romantically interested in Kojima, though he does not share her feelings. She considers Kanon to be her rival for Kojima's affections. ;Suzuki A schoolmate and close friend of Kanon. ;Miyako Miyamori Kanon's mother, who typically sports a carefree attitude, as well as a penchant for seeing opportunities for money-making. She uses a special golden ring to make delicious pickles, in a recipe passed down in the family. ;Sedna An evil spirit sealed away in part on the island and in part in the sea. The seal on Sedna is broken as Marin and Urin are trying to retrieve Kanon's ring. ;Ichikawa Matsumoto's daughter. Unlike her father, Ichikawa does not speak a human language. She enjoys Miyako's pickles. ;Utasha
Manga
Two manga adaptations of Umi Monogatari were serialized in two different magazines by different publishers. The first one, with the title Umi Monogatari ~Anata ga Ite Kureta Koto~, follows the same story as the anime. The second, titled Umi Monogatari: Minna Aishiteru!, has a yonkoma format and follows Marin and Urin's experiences on the island. The first manga, illustrated by Akira Katsuragi, was serialized in Mag Garden's Monthly Comic Blade magazine. The second manga, illustrated by Tonmi Narihara, was serialized in Futabasha's Comic High! magazine.
Music
The anime uses two theme songs. The opening theme is "violet" by Marble and the ending theme is "Tōmei na Inori" by Masumi Itō. Lantis has released three soundtracks for Umi Monogatari ~Anata ga Ite Kureta Koto~. The first, "violet", was released on July 23, 2009. The second, "Treasure!? ~Kimi to Deaeta koto~", was released on August 5, 2009. The third, "Tōmei na Inori", was released on August 26, 2009.
