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Aika Sumeragi, a high school age C-Class salvager, is hired by class president Eri Shinkai to pilot her salvaging vessel. After introducing Aika to the treasure hunting club, and revealing coordinates hidden in a marking on fellow classmate Karen Minamino's chest, they embark on an adventure to see where it leads.

After arriving at the coordinates, Eri decides the treasure hunting club needs a break and declares that they will have a beach party on a nearby island. They are not alone, however, and Aika has to fight her teacher to protect Karen.

In the final episode of the series, Aika, Eri, and Karen find an old airport at the bottom of the ocean. After exploring the still functional facility, they finally uncover the secret to Karen's past.