When a mysterious villain threatens to open the gates of Hell, a devilishly handsome demon hunter could be the world's best hope for salvation.

Kaede Komura is in love. After being seated next to the airheaded Ai Mie for the past three days, Komura cannot help but be attracted to his bespectacled classmate. Although he has yet to have a proper conversation with Mie, Komura dreams of the day when she will look his way. One day, Komura notices that his seatmate is not wearing her glasses, learning by questioning Mie that she forgot them and has a tendency of doing so. When he sees her struggle, a concerned Komura takes it upon himself to help his crush. As the boy offers his assistance to Mie day by day, her reasons for constantly losing her glasses slowly evolve from the answer she initially gave.

After witnessing his mother's death at a young age, Uryuu Kamihate grew up disillusioned with the gods, dismissing faith in them as utter nonsense. Due to his beliefs, he aims to become a doctor, spending most of his time studying to enter the prestigious Kyoto University. To further his studies, Uryuu searches for a new place to take him in and get away from the noisy environment of the orphanage where he grew up. Chidori Amagami, head priest of the Amagami Shrine, allows him to stay at the Amagami residence. However, Uryuu has to live there with Chidori's three granddaughters—Yae, Yuna, and Asahi—who serve as shrine maidens. Moreover, Chidori has another condition for Uryuu: he has to marry one of the sisters to ensure the shrine's future and one day inherit it. But with Uryuu's goals and outlook naturally clashing against the sisters' beliefs, this already daunting ordeal seems outright impossible.

Touya Mochizuki grows accustomed to his new life in another world. Armed with his trusty smartphone, the teenager accepts small quests at his leisure while spending time with his new fiancées: Yumina Urnea Belfast, Yae Kokonoe, and twin sisters Linse and Elze Shileska. But even in a relaxing environment, Touya is only beginning to understand the responsibilities that come with these engagements. In addition to his romantic woes, the elder fairy Lean wishes to locate the remaining pieces of Babylon, the floating island that the mysterious Professor Regina Babylon created five thousand years ago. Touya reluctantly accepts her request and seeks out the teleportation circles that will lead to the islands. However, strange monsters have emerged—possibly powerful enough to destroy the world. In order to save the second life he was given, Touya must weaponize ancient technologies to fight these monsters—all while balancing his overwhelming number of relationships.