Boyle Samejima is a tough-as-nails cop with a love for hard-boiled action. He's so hard-boiled that his inner-monologue boxes have blood splatters on them! But after he punches a suspect in the middle of a hostage situation, he's shipped off to the remote island of Anegashima near Okinawa. Soon after he arrives, he learns that this island is anything but quiet and peaceful… The precinct is in the middle of an investigation involving the mass disappearance of a cult that worshipped a little girl who is said to have been raised by a dolphin, and that little girl is in their custody. But when Samejima arrives for his first day of work, he's faced with the shock of his life! His new partner, the father of that little girl, really is a dolphin! It's okay, though, the dolphin can walk on land and has two hands...and can talk!
Ocean crime is on the rise and it’s keeping the Anegashima police department very busy. Samejima and Orpheus get a personal call from Captain Jinguji of the mysterious Foreign Ocean Affairs Section Seven’s Mobile Investigations Unit. According to her sources, several members from the ocean gang known as the Osawara Family are on the move. And many of these bad-boy fishmen have it out for Orpheus, including one of their higher-ups, Kinnosuke Kaji. But what could this fiendish marlin be after on land? And more trouble is brewing when Jura accuses her boss—a former member of the Cult of the Sea—of trying to murder her coworker and friend Yukari!
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!
Samejima, Orpheus, and the MIU agents continue their investigation by diving into undersea ruins. But they aren’t alone... Kamuro and his gang are there too, along with some of the baddest and roughest mermen yet! Hope for victory dwindles when Samejima is poisoned in battle and Chako is kidnapped by cult members while in police custody on land. Meanwhile, Kamuro inches ever closer to his prize, the Left Hand of Poseidon.
The battle in the undersea ruins continues. Chako is brought into the midst of the fighting and her presence allows the inner sanctum to be opened, revealing the weapon that Kamuro has been searching for: the hand of Poseidon! And with it comes the power to sink the whole world into the depths of the sea. In this final confrontation, the history of how Orpheus and Kamuro went from friends to bitter enemies is revealed… Don’t miss the exciting and heart-wrenching conclusion of Hard-Boiled Cop and Dolphin!