"Owari no Seraph" is set in a post-apocalyptic world where a mysterious virus has wiped out most of humanity, leaving only children under the age of 13 unaffected. In the aftermath, vampires emerge from the shadows and establish dominance over the surviving human population, enslaving them and using them as livestock for blood. The story centers on Yuichiro Hyakuya, a young boy who loses his family and friends during the initial outbreak and is subsequently captured and raised in a vampire-controlled orphanage alongside other children, including his close friend Mikaela Hyakuya. Yuichiro harbors a deep hatred for vampires and dreams of escaping to exact revenge. Mikaela, however, devises a risky plan to help the children escape, which ultimately fails, resulting in Mikaela's apparent death and Yuichiro's capture by the Japanese Imperial Demon Army, a human military organization dedicated to fighting vampires. Years later, Yuichiro becomes a soldier in the Demon Army, wielding a cursed weapon known as a Demon Cursed Gear, which grants him enhanced abilities to combat vampires. He joins a squad of fellow soldiers, including Shinoa Hīragi, Yoichi Saotome, and Shihō Kimizuki, each with their own motivations and tragic pasts. As Yuichiro and his squad engage in battles against the vampires, they uncover dark secrets about the world and their own organization. The vampires, led by noble families such as the Progenitors, are revealed to have their own complex hierarchy and motivations, with some factions seeking to maintain control over humans while others pursue more sinister goals. Mikaela, thought to be dead, reappears as a vampire, creating tension as Yuichiro struggles to reconcile his hatred for vampires with his lingering bond to his childhood friend. The narrative explores themes of loyalty, revenge, and the blurred lines between humanity and monstrosity. Yuichiro's journey is marked by his internal conflict between his desire for vengeance and his growing connections to his comrades. The series also delves into the origins of the virus, the true nature of the Demon Cursed Gear, and the hidden agendas of both the Demon Army and the vampire aristocracy. As the story progresses, alliances shift, and the characters are forced to confront the moral ambiguities of their actions in a world where survival often comes at a cost.