Having witnessed a spectral presence in the aquarium as a child, Ruka finds herself irresistibly drawn back to the aquatic world where her father is employed. Here, she encounters two enigmatic boys, Umi and Sora, who share her unique connection to the sea and its mystical echoes. Raised by dugongs, the trio embarks on a journey intertwined with the eerie vanishing of marine life, unbeknownst to the adult caretakers at the aquarium who remain oblivious to the unfolding enigma.