"Abunai Danjo Kousai" centers on the unconventional relationship between two high school students, Kyouichi and Rika, who are step-siblings. After their parents remarry, the two are forced to live under the same roof, despite their initial discomfort and mutual dislike. Kyouichi, a reserved and serious student, struggles to adjust to his new family dynamic, while Rika, outgoing and mischievous, takes pleasure in teasing him. Their interactions are marked by a blend of tension and humor as they navigate their new living situation.

The story explores the complexities of their relationship, which gradually evolves from antagonistic to something more ambiguous. Rika’s playful and flirtatious behavior often puts Kyouichi in awkward situations, testing his patience and resolve. Despite his attempts to maintain boundaries, Kyouichi finds himself increasingly drawn to Rika, creating a dynamic that blurs the line between sibling affection and romantic interest. The narrative delves into themes of family, identity, and the challenges of adolescence, as both characters grapple with their feelings and the societal expectations surrounding their relationship.

Supporting characters, such as their classmates and friends, add depth to the story, providing additional perspectives and complicating the central relationship. The manga balances lighthearted moments with more serious undertones, creating a narrative that is both engaging and thought-provoking. Through its exploration of the evolving bond between Kyouichi and Rika, "Abunai Danjo Kousai" offers a nuanced portrayal of the complexities of human relationships and the emotional turbulence of youth.