"Gyouten Ningen Batsealer" is a science fiction anime series set in a futuristic world where humanity faces extinction due to the invasion of monstrous creatures known as "Bats." These creatures, which resemble giant bats, have overrun the Earth, forcing the remnants of humanity to live in isolated, fortified cities. The story centers on a group of genetically enhanced warriors called Batsealers, who are humanity's last line of defense against the Bats. These warriors are equipped with advanced technology and possess unique abilities derived from their genetic modifications, allowing them to combat the Bats effectively.

The protagonist, a young man named Ryu, is one of the most skilled Batsealers. Ryu is driven by a personal vendetta against the Bats, as they were responsible for the death of his family. Throughout the series, Ryu's journey intertwines with other Batsealers, each with their own motivations and tragic pasts. Among them is Aya, a determined and resourceful fighter who becomes Ryu's closest ally, and Kaito, a former scientist whose experiments inadvertently contributed to the creation of the Bats. Together, they form a team dedicated to eradicating the Bats and reclaiming the Earth for humanity.

The narrative explores themes of survival, sacrifice, and the ethical implications of genetic engineering. As the Batsealers venture beyond the safety of their cities to confront the Bats, they uncover dark secrets about the origins of the creatures and the true nature of their own enhancements. The series delves into the moral dilemmas faced by the characters, particularly Kaito, who struggles with guilt over his role in the catastrophe. The plot is driven by intense battles, strategic missions, and the gradual unraveling of the mystery behind the Bats' emergence.

As the story progresses, the Batsealers encounter increasingly powerful and intelligent Bats, leading to escalating conflicts that test their resolve and unity. The series also examines the fragile alliances between the remaining human factions, highlighting the tension between cooperation and self-preservation in a world on the brink of collapse. The climax of the series sees Ryu and his team confronting the ultimate source of the Bats' power, culminating in a decisive battle that will determine the fate of humanity.