The story begins with Hayato Kumakura waking up disoriented on a cold stone floor, realizing he's in the summoning chamber where he was brought to this world three years prior. He recalls being summoned from Japan to a medieval-level world to help defeat the Demon Lord, a process that occurs only once every ten years. As the youngest among the summoned, Hayato possesses enhanced physical abilities comparable to a brown bear, holy magic for healing, and a unique spatial storage ability. His powers were instrumental in the empire's victory, but now that the war is over, the Prime Minister reveals Hayato's purpose is fulfilled. To avoid the complications of breaking promises of promotion, land, and marriage, the Prime Minister arranges for Hayato to be sent back to his original world, a fate he accepts to avoid potential death in the empire. He is then forcibly returned to Japan, finding himself back in his high school classroom.
Upon returning, Hayato discovers that three years have passed in Japan as well, and the world is in chaos. He encounters a zombie and quickly dispatches it with his spear, confirming his knowledge of them from the other world. Investigating the school's staff room, he finds a file detailing a global zombie outbreak that began around the time of his summoning. The virus is believed to have spread through the atmosphere, possibly amplified by Earth's ambient magic, which is absent in normal human biology. The file describes the collapse of society, the inability to control the zombies, and the eventual breakdown of infrastructure and order.
Hayato then ventures into the deserted city, his spatial storage filled with military supplies from the other world. He stops at a ransacked supermarket, fighting more zombies to search for supplies, but finds little of use. He decides to head to his home, finding it looted. He then encounters Yui Shiraishi, a girl who has been bitten by a zombie and is awaiting her transformation. Hayato, possessing holy magic capable of healing the infected before they fully turn, offers to cure her in exchange for information and cooperation. Yui, ostracized from her shelter, agrees, though she initially mistakes Hayato's intentions due to his perceived interest in her young appearance.
Hayato and Yui decide to use Yui's house as a base, as his own home was more severely damaged. Hayato's immediate goal is to find a car, as their current supplies will eventually run out and their current location is unsafe. He believes a reliable vehicle is necessary for long-term survival and to potentially find a more secure location. He recalls his high school friend attending Kirigaoka Agricultural High School, a place with extensive grounds and resources, including water and farmland, which he believes could be a viable option for survival.
Hayato sets out on his bicycle to find a car, eventually locating a suitable SUV in a wealthy residential area. However, the car's battery is dead. He then travels to Kirigaoka Agricultural High School, hoping to find someone who can teach him how to replace a car battery. He approaches the school, which is fortified with barricades, and encounters two female students. He explains his situation and offers salt from his spatial storage in exchange for knowledge. The students, impressed by his offer and the potential for self-sufficiency, agree to help him meet with the school's teachers. He is eventually brought before the vice principal, Omori, to negotiate.
