A world where it has been established that people routinely swing between slightly different parallel worlds... Koyomi Hidaka, who lives with his father following his parents' divorce, meets a girl named Shiori Satou at the research institute where his father works. They both have a little crush on each other, but everything changes when their parents remarry. Koyomi and Shiori, convinced that they will never be together, try to jump into another world where they won't be step-siblings... There's no meaning in a world without her.

It is not strange that the Demon Lord's forces fear the Crimson Demons, the clan from which Megumin and Yunyun originate. Even if the Demon Lord's generals attack their village, the Crimson Demons can just easily brush them off with their supreme mastery of advanced and overpowered magic.

When Yunyun receives a seemingly serious letter regarding a potential disaster coming to her hometown, she immediately informs Kazuma Satou and the rest of his party. After a series of wacky misunderstandings, it turns out to be a mere prank by her fellow demon who wants to be an author. Even so, Megumin becomes worried about her family and sets out toward the Crimson Demons' village with the gang.

There, Kazuma and the others decide to sightsee the wonders of Megumin's birthplace. However, they soon come to realize that the nonsense threat they received might have been more than just a joke.

"Mushi": the most basic forms of life in the world. They exist without any goals or purposes aside from simply "being." They are beyond the shackles of the words "good" and "evil." Mushi can exist in countless forms and are capable of mimicking things from the natural world such as plants, diseases, and even phenomena like rainbows. This is, however, just a vague definition of these entities that inhabit the vibrant world of Mushishi, as to even call them a form of life would be an oversimplification. Detailed information on Mushi is scarce because the majority of humans are unaware of their existence. So what are Mushi and why do they exist? This is the question that a "Mushishi," Ginko, ponders constantly. Mushishi are those who research Mushi in hopes of understanding their place in the world's hierarchy of life. Ginko chases rumors of occurrences that could be tied to Mushi, all for the sake of finding an answer. It could, after all, lead to the meaning of life itself. [Written by MAL Rewrite]

Love has never really been a concern for Mitsuyoshi Tada, and as the aspiring photographer enters his second year of high school, it truthfully couldn't be further from his mind. However, things just might change after he meets a bright and bubbly foreigner named Teresa Wagner while he was taking pictures of a cherry blossom tree. Nevertheless, after she asks him to photograph her, the two soon separate... only to meet each other again twice more that same day! Finding Teresa just as she is caught in a sudden downpour, Tada invites her to his family's coffee shop to dry off. There, she explains that she was separated from her traveling companion, a no-nonsense redhead named Alexandra ''Alec'' Magritte. When Alec reunites with Teresa shortly after, they say their goodbyes, expecting to part ways for good—but the two unexpectedly show up as transfer students in his class the next day. Teresa and Alec quickly get used to their lives at Koinohoshi High School and decide to join Tada in the photography club, along with his narcissistic friend Kaoru Ijuuin, the idol-obsessed Hajime Sugimoto, serious class rep Hinako Hasegawa, and the dog-like Kentarou Yamashita. With these two peculiar additions to his equally eccentric group of friends, Tada's second year of high school is about to get even livelier, and he might need to start rethinking his approach to love. [Written by MAL Rewrite]