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Shimako, a socially anxious office worker, escapes the pressures of her company's beach retreat by fishing alone, only to stumble upon a trapped turtle by the shore. When she rescues it, Shimako is mysteriously whisked away to a hidden underwater palace, where she meets Oto-hime, a breathtakingly beautiful but enigmatic princess. Drawn in by Oto-hime's charm and her lavish hospitality, Shimako soon realizes there is something unsettling beneath the princess's kind façade.