"Rainbow - Nisha Rokubō no Shichinin" is set in 1955 Japan and revolves around seven teenage boys incarcerated in Shōnan Special Reform School, a harsh juvenile detention facility. The story begins with the arrival of Mario Minakami, a reserved yet determined boy, who quickly forms a bond with five other inmates: Joe Yabuki, a fiery and rebellious boxer; Noboru Maeda, a timid and intellectual boy; Tadayoshi Sakuragi, a former judo champion; Jōji Sasaki, a quiet and resourceful individual; and Kyōichi Hatanaka, a street-smart and loyal friend. They are later joined by Rokurouta Sakuragi, a former medical student and older inmate who becomes their mentor and protector.

The boys endure brutal treatment from the sadistic guards, particularly Ishihara, who takes pleasure in tormenting them. Rokurouta, nicknamed "An-chan," inspires the group to resist their oppressors and maintain their humanity despite the dehumanizing conditions. He teaches them the value of friendship, resilience, and hope, urging them to dream of a life beyond the prison walls. The boys form a tight-knit bond, vowing to survive and reunite after their release.

As the narrative progresses, each character’s backstory is revealed, shedding light on the circumstances that led to their incarceration. Mario’s tragic past, Joe’s struggles with poverty and violence, Noboru’s intellectual aspirations, Tadayoshi’s fall from grace, Jōji’s resourcefulness, and Kyōichi’s loyalty are explored in depth. Rokurouta’s own story, marked by betrayal and loss, adds emotional weight to his role as the group’s moral compass.

The series delves into themes of systemic oppression, the resilience of the human spirit, and the enduring power of friendship. The boys face numerous challenges, including physical abuse, psychological manipulation, and the constant threat of death. Their journey is one of survival and self-discovery, as they navigate the harsh realities of their environment while clinging to the hope of a better future.

After their eventual release, the boys must confront the challenges of reintegrating into society, each pursuing their own path while staying true to the bond they formed in Shōnan. The story concludes with a poignant reflection on their shared experiences and the enduring impact of their time together. "Rainbow - Nisha Rokubō no Shichinin" is a gritty and emotionally charged narrative that explores the depths of human suffering and the strength of camaraderie in the face of adversity.