Aika Sumeragi, a sixteen year old high school student, offers her services as a C-class Salvager by spreading posters at school after getting her C-class Salvager license. She is immediately hired by Eri Shinkai, Aika’s wealthy class president and founder of the Treasure Hunting Club. After being introduced to Karen Minamino, a mysterious girl with unknown background and a butterfly insignia on her left breast, the club starts an oceanographic expedition on Eri’s private cruiser to uncover the riddles encoded in Karen’s insignia.
During their journey more questions arise as Risako Nagisa, the schools form teacher, attempts to sabotage the expedition and capture Karen, and a suspicious underwater airport is found by the team with Aika’s beloved Beluga submarine. While the pieces of the puzzle slowly come together, Karen’s life is turned upside down when the team discovers ancient technology to which Karen is the key and traces of biogenetic experiments.
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