When Demon Queen Echidna begins her invasion of the human realm, the people turn to the hero Leo Demonheart to protect them. Blessed with insurmountable power, Leo easily repels Echidna's army, thereby saving the world. But instead of gratitude or admiration, Leo is met with disdain and scorn from his fellow humans, who fear his overwhelming strength and believe that it may eventually cause humanity's demise. Banished from the kingdom he once called home, Leo wanders aimlessly throughout the land until, one day, he hears rumors of Echidna's efforts to rebuild her army. Hoping that demonkind might accept him as an ally, he returns to the demon queen's castle and offers Echidna his help. Naturally, Echidna immediately rejects him. Still, not all hope is lost, as Leo manages to convince Echidna's four generals to let him secretly work under their supervision. Donning the identity of a masked dark knight named Onyx, Leo solves the demon army's problems one by one—gradually improving the quality of life around the castle. Above all else, however, Leo wants to accomplish one goal: to learn why Echidna started the war in the first place.

A large-scale catastrophe is occurring across the planet. Ability users are discovered after the appearance of a mysterious fog, apparently having committed suicide, so the Armed Detective Agency sets out to investigate these mysterious deaths. The case seems to involve an unknown ability user referred to as Collector, a man who could be the mastermind behind the incident. Trust and courage are put to the test in order to save the city of Yokohama and ability users across the world from the grip of Collector where the Armed Detective Agency forms an unlikely partnership with the dangerous Port Mafia.

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.

Satou Matsuzaka is a beautiful high schooler who has a reputation for being permissive with men. However, a chance encounter with a young girl named Shio Koube makes Satou realize that this is her first and only true feeling of love. Telling others that she lives with her aunt, Satou secretly shares an apartment with Shio. Despite her innocent appearance, Satou is willing to do anything to protect her beloved, resorting to desperate measures to ensure that their happy sugar life remains intact.