Yukari Okina has vanished into the sunken ruins off the coast of Anegashima. Public safety officers declare her dead and pin everything on her friend Hamura. Hamura tries to tell them that Kamuro and the Cult of the Sea are behind the disappearance, but no one is willing to hear her out—except for Samejima, who is determined to help her. Soon after, the Marine Mobile Investigations Unit arrives on the scene and quickly recruits Samejima and Orpheus to begin their own investigation!