Kazuma Satou, former NEET and current reincarnated adventurer, finally returns home after the incident at the Crimson Demon village. He is joined by his ever-reliable companions: self-absorbed goddess Aqua, one-trick pony arch-wizard Megumin, and masochistic crusader Lalatina Ford "Darkness" Dustiness. Following their recent exploits, the party is one step closer to vanquishing the Demon King and his armies—a terrifying task that Kazuma is eager to ignore for as long as possible. Things, however, are not going as smoothly as the group would like; they are constantly racking up debt, getting sidetracked during missions, and falling into traps due to their recklessness and eccentricities. As the party struggles with their perennial issues, they receive an invitation from a princess interested in hearing tales of the group's "heroic" deeds. Darkness' pleas to refuse the invitation only fire the others up to accept it, marking the start of yet another uncanny adventure with unforeseen consequences.

Tensho School is located in the 7th Ward of Thrive Central, where emphasis is put on extracurricular activities and all clubs participate in an annual fall singing contest. Having to delay the start of his school year due to his older sister's hospitalization, transfer student Saku Otonomiya finds himself struggling to adapt to his hectic new life. Balancing his part-time job and caring for his sister, he finds it difficult to make time to join a school club. In preparation for the upcoming singing contest, the school's singing club wishes to use Sousuke Kagura's song "Cloudy Days," but Kagura is reluctant to grant his permission. The song requires someone with a three-octave range to sing it and he believes that only a VOCALOID can meet the standards. He allows for the club to use his song on the condition that they can find someone who possesses that vocal range to properly sing it. While racking his brain over the composition of his next song, Kagura stumbles upon Otonomiya singing "Cloudy Days" perfectly. He invites Otonomiya to form a unit with him for the competition. At the same time, the singing club tries to goad him into joining so they can use Kagura's song. As the contest inches closer, Otonomiya must overcome his uncertainty and decide who to team up with.

Megumin is a young and passionate wizard from the Crimson Demon Village, a remote community of mages with red eyes and a flair for the dramatic. She has devoted her life to mastering explosion magic, a powerful but impractical spell that leaves her drained of mana and unable to move for the rest of the day. Regardless, she refuses to learn any other skills. Along with her childhood friend and self-proclaimed rival, Yunyun, Megumin enrolls in the Red Prison: a prestigious academy for Crimson Demon magic users. There, she learns more about the secrets and history of her clan, as well as the threats and challenges they must face. As she polishes her power at the Red Prison with the help of her loyal familiar Chomusuke and her adorable little sister Komekko, Megumin aims to become the greatest explosion wizard of all time!

In the town of Kurouzu-cho, Kirie Goshima lives a fairly normal life with her family. As she walks to the train station one day to meet her boyfriend, Shuuichi Saito, she sees his father staring at a snail shell in an alley. Thinking nothing of it, she mentions the incident to Shuuichi, who says that his father has been acting weird lately. Shuuichi reveals his rising desire to leave the town with Kirie, saying that the town is infected with spirals. But his father's obsession with the shape soon proves deadly, beginning a chain of horrific and unexplainable events that causes the residents of Kurouzu-cho to spiral into madness.