"Gyo" is a horror manga that explores a surreal and grotesque scenario where marine life becomes a source of terror. The story begins with Tadashi, a young man vacationing in Okinawa with his girlfriend, Kaori. Their peaceful trip is disrupted when a strange, foul-smelling fish with mechanical legs emerges from the ocean. This bizarre creature is the first sign of a larger, more horrifying phenomenon: marine life across Japan begins to invade land, equipped with mechanical legs and emitting a noxious gas. The gas, later revealed to be a weaponized bacterial strain developed during World War II, infects living organisms, turning them into grotesque, decaying hosts for the mechanical creatures.

As the infestation spreads, Tadashi and Kaori flee to Tokyo, where the situation escalates. The fish, now accompanied by larger sea creatures, overrun cities, causing widespread panic and destruction. Kaori becomes increasingly paranoid and physically ill, her body reacting to the pervasive stench of the gas. Tadashi struggles to protect her while uncovering the truth behind the outbreak. He learns that the gas is a product of wartime experiments, designed to create biological weapons, and that the mechanical legs were developed by a scientist named Dr. Koyanagi, who sought to weaponize marine life.

The narrative takes a darker turn as the gas begins to affect humans, transforming them into monstrous, decaying beings controlled by the mechanical creatures. Kaori succumbs to the infection, becoming one of these grotesque entities. Tadashi, devastated by her transformation, continues his journey, encountering other survivors and remnants of the military attempting to combat the outbreak. The story culminates in a chaotic and surreal confrontation, with Tadashi facing the horrifying reality of a world overrun by the infected and the relentless march of the mechanical sea creatures.

The manga delves into themes of biological warfare, environmental disaster, and the fragility of human life, presenting a bleak and unsettling vision of humanity's vulnerability. The characters, particularly Tadashi and Kaori, are portrayed as ordinary individuals thrust into extraordinary circumstances, their struggles and transformations serving as a lens through which the horror of the narrative unfolds. The plot is driven by a sense of escalating dread, with the grotesque imagery and unsettling atmosphere reinforcing the story's themes of decay and destruction.