"Hard-Boiled Cop and Dolphin" is a manga series that blends elements of crime, action, and science fiction. The story is set in a near-future world where humanity faces a new threat: the emergence of "Aqua-Evolved Humans," individuals who have undergone a mysterious transformation that grants them aquatic abilities but also drives them to commit violent crimes. These Aqua-Evolved Humans are hunted by a specialized police unit tasked with maintaining order and protecting society from their unpredictable behavior.

The protagonist, Ryuusei Samejima, is a hardened detective with a reputation for his ruthless methods and unyielding dedication to justice. His life takes a dramatic turn when he is assigned to work alongside a unique partner: a highly intelligent, cybernetically enhanced dolphin named Ui. Ui, equipped with advanced technology and capable of human speech, serves as both a tactical asset and a moral counterbalance to Samejima's often brutal approach. Together, they form an unlikely duo, navigating a world where the line between human and monster becomes increasingly blurred.

The narrative explores the complexities of their partnership as they investigate cases involving Aqua-Evolved Humans, uncovering the dark secrets behind their transformation. The series delves into themes of identity, morality, and the consequences of scientific experimentation, as Samejima and Ui confront not only external threats but also their own internal struggles. The dynamic between the stoic, emotionally scarred detective and the empathetic, logical dolphin creates a compelling contrast, driving both character development and plot progression.

As the story unfolds, the duo faces increasingly dangerous adversaries, including rogue Aqua-Evolved Humans and shadowy organizations with hidden agendas. The stakes escalate as they uncover a conspiracy that threatens to destabilize society, forcing them to question their roles in a world where humanity's survival hangs in the balance. The series combines intense action sequences with moments of introspection, offering a balanced exploration of its central themes while maintaining a fast-paced, engaging narrative.