Love has never really been a concern for Mitsuyoshi Tada, and as the aspiring photographer enters his second year of high school, it truthfully could not 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, the 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.

The Demon Lord Tasogare's castle is a dark and frightening place, filled to the brim with various monsters. Any soul unfortunate enough to be imprisoned here is sure to be terrified by the horrors within. However, the human princess Aurora Suya Rhys "Syalis" Kaymin is a different case. Rather indifferent to her situation, Syalis worries about one thing and one thing only—sleep. Ever since the demon lord kidnapped her from her kingdom, she has not had a single good night's rest. To alleviate her dozen dozing issues, the princess makes do with what she can find in the castle. Whether it be the fur of fluffy demonic teddy bears or the silky, blanket-like bodies of ghost shrouds, everything is but a means to ensure a peaceful slumber. With so many potential materials to craft items that can help her sleep at her disposal, nothing will stop the sleepy princess—not even death.

Shin Koihime†Musou characters sing theme songs on stage at a live event.

When the sky turns red, the moon turns black, and monsters begin roaming the streets, Satsuki Kakeru is at a loss for what to do. Along with his best friend Yuka, they try to decipher why they have been sent to this strange world, which is seemingly empty aside from themselves. However, when the "Red Night" ends, Kakeru and Yuka believe it was all a dream—until it happens again, and they are left in a dangerous situation. They meet four others in the same predicament: Kusakabe Misuzu, an expert swordswoman; Tachibana Kukuri, a strange mute girl who looks uncannily like Kakeru's deceased sister; Hirohara Yukiko, a lively young girl whose personality reverts to that of a cold killer when her glasses are removed; and Tajima Takahisa, a young pyrokineticist. As the six of them band together to survive and discover what this mysterious world is, things take a turn for the worse as six shadows appear before them...