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.

Kanon Miyamori is looking forward to Summer vacation, and explains that she will be introducing the viewer to recommended locations on the island.

Kanon tells the viewer about the local shrine and a romantic dating spot.

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.

Kanon discusses her daily routine and goes for a walk.

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.

Kanon's childhood friend, Suzuki, talks about their friendship.

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.

While the tide is low, Kanon visits Kagero Island and goes exploring.

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.

Kanon invites the viewer into her house for tea and a brief tour.

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.

Ooshima talks about how she is the ideal woman of Amamiko Island.

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.

Kanon takes us to the beach, but explains that she doesn't actually like going there.

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.

The sea turtle, Ichikawa, talks briefly about herself and her family.

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.

Marin takes us below the waves to talk about her friends who live there.

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Kanon explains that sometimes a good shout or fume makes you feel better. We all have stress that builds up.

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It's Suzuki's birthday tomorrow! Are you going, Kanon?

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Summer is nearly over, Kanon and Marin both hope you'll visit again soon!

Marin and the girls try to make a boxed lunch for Kojima.

Marin borrows Kanon's school uniform and visits her high school.

Marin and the girls have a pajama party.

Ooshima tries to teach the girls how to be maids.

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