"Dein und mein Geheimnis" centers on the unconventional relationship between Akane Hino, a high school student, and her adoptive older brother, Shigure Nanamine. The story begins with Akane harboring a deep crush on Shigure, who is not only her brother but also a popular actor. Despite their sibling status, Shigure reciprocates Akane's feelings, leading to a complex and emotionally charged dynamic. The narrative explores their struggles as they navigate societal norms, familial expectations, and their own conflicting emotions.

Shigure, known for his aloof and mysterious demeanor, hides a darker side beneath his charming public persona. His possessiveness and manipulative tendencies create tension in his relationship with Akane, who is torn between her affection for him and her desire for a normal life. As their bond deepens, Akane begins to uncover unsettling truths about Shigure's past, including his traumatic childhood and the reasons behind his adoption into her family. These revelations force her to confront the moral and emotional complexities of their relationship.

The story also introduces secondary characters who influence the central narrative. Akane's classmates and friends provide contrasting perspectives on love and relationships, while Shigure's colleagues in the entertainment industry add layers to his character, revealing the pressures he faces as a public figure. The interplay between these characters highlights the challenges Akane and Shigure face in reconciling their feelings with the expectations of those around them.

Throughout the series, themes of love, identity, and the consequences of trauma are explored. The narrative delves into the psychological impact of Shigure's past on his present behavior, as well as Akane's growth as she grapples with her feelings and the ethical dilemmas posed by their relationship. The story balances moments of tenderness and humor with darker, more introspective elements, creating a multifaceted portrayal of its central characters and their evolving connection.