"Zeroin" is a manga series that centers on a dystopian future where humanity is on the brink of extinction due to a catastrophic event known as the Collapse. The world is overrun by monstrous entities called Aberrants, which are the result of genetic experimentation gone wrong. In this bleak setting, the remnants of humanity survive in fortified cities, relying on advanced technology and specialized soldiers known as Reclaimers to protect them.

The protagonist, Kael Ardent, is a skilled Reclaimer who harbors a deep personal vendetta against the Aberrants. His younger sister, Lina, was killed during an Aberrant attack, fueling his relentless drive to eradicate the creatures. Kael is known for his exceptional combat abilities and his use of a unique weapon called the Zeroin, a high-tech device capable of neutralizing Aberrants by disrupting their genetic structure. However, the Zeroin is a double-edged sword, as its use comes at a physical and mental cost to the wielder.

As the story progresses, Kael discovers that the Aberrants are not mindless monsters but are instead the result of a failed government project aimed at creating superhuman soldiers. This revelation complicates his mission, as he begins to question the morality of his actions and the true nature of the enemy. Alongside his comrades, including the resourceful engineer Mira and the enigmatic former scientist Dr. Elias, Kael uncovers a conspiracy that reaches the highest levels of the surviving human government.

The narrative explores themes of survival, morality, and the consequences of unchecked scientific ambition. Kael's journey is marked by intense battles, moral dilemmas, and the gradual unraveling of the truth behind the Collapse. The series balances action-packed sequences with moments of introspection, as Kael grapples with his own humanity and the blurred lines between friend and foe. The world-building is detailed, with a focus on the technological and societal adaptations humanity has made to survive in a hostile environment. The Aberrants are depicted as both terrifying and tragic, adding depth to the conflict and challenging the characters' perceptions of good and evil.