"The Legend of Black Heaven" centers around Oji Tanaka, a middle-aged salaryman who once led a double life as the guitarist for a rock band called Black Heaven. In his youth, Oji, known by his stage name Gabriel, pursued his passion for music with fervor, but after marrying and having a child, he abandoned his rockstar dreams to focus on providing for his family. His mundane office job and strained relationship with his wife, Kyoko, who disapproves of his past lifestyle, leave him feeling unfulfilled.

The story takes a dramatic turn when Oji encounters Layla Yuki, a mysterious woman who reveals that his guitar riffs possess a unique energy capable of powering an alien weapon system. This weapon is crucial in an ongoing interstellar war against a hostile alien race. Layla recruits Oji to play his music once more, as his sound is the key to activating the weapon and saving humanity. Reluctantly, Oji agrees, reigniting his passion for music while balancing his responsibilities as a father and husband.

As Oji becomes more involved in the alien conflict, his dual life creates tension at home. Kyoko grows suspicious of his late-night absences and erratic behavior, unaware of the extraordinary circumstances. Meanwhile, Oji struggles with the moral implications of using his music as a weapon, questioning whether his art should serve destructive purposes. The series explores themes of identity, sacrifice, and the enduring power of creativity, as Oji navigates his roles as a musician, father, and unlikely hero.

The supporting cast includes Layla, who serves as the bridge between Oji and the alien forces, and Gen, Oji's former bandmate, who provides comic relief and occasional assistance. The aliens, portrayed as both allies and adversaries, add layers of complexity to the narrative, as their motivations and the true nature of the war are gradually revealed. The series blends elements of science fiction, comedy, and drama, creating a unique story that juxtaposes the mundane struggles of adulthood with the extraordinary stakes of an interstellar conflict.