Tamao Hiroi, a former member of his high school baseball team, returns home one day to find someone in his room. It is Ran Homura, the school's most beautiful girl and the ace of the softball team. He watches as she masturbates on his bed while whispering his name, and he is caught. This embarrassing encounter takes an even more complicated turn when their parents announce they are remarrying, forcing Tamao and Ran to become step-siblings living under the same roof.

Rather than let the secret destroy her reputation, Ran takes control of the situation. She uses a threat to accuse Tamao of attempted assault to blackmail him, establishing a strange master and slave relationship where he must obey her commands. Her demands are primarily sexual in nature, and a deeply immoral double life begins between them. However, it becomes clear that Ran has genuinely liked Tamao for a long time. Her aggressive, dominant behavior is a tsundere facade to hide her true feelings of embarrassment from being caught, and her threats are a way to initiate intimacy without admitting her affection.

As their secret games progress, their relationship deepens beyond mere physicality. Ran becomes more openly affectionate, calling Tamao her older brother and seeking skinship. The boundaries between master and slave and genuine romantic feelings begin to blur. Their strange relationship also has a positive effect on Tamao, helping him begin to revive his lost passion for baseball. The main characters are Tamao Hiroi, a talented artist who became disillusioned with baseball, and Ran Homura, the possessive and intense softball ace who secretly admires him. The setting is primarily their shared home and high school. A notable narrative arc involves the introduction of a love rival, a female manager of the baseball club who secretly likes Tamao, which triggers jealousy in Ran and creates a classic love triangle. The story explores the intertwining of sexual awakening, blackmail, and emerging love between two teenagers navigating their new family situation and unresolved personal feelings.