In a near-future Japan, the government enacts the "Red Threads of Science" law to combat declining birth rates, mandating that citizens aged 16 to 18 marry partners assigned by the state based on compatibility algorithms. Yukari Nejima, a high school student, has harbored feelings for his childhood friend, Misaki Takasaki, for years. Just as he prepares to confess his love, he receives his government-assigned marriage notification, pairing him with Ririna Sanada, a reserved and enigmatic classmate. Despite the arrangement, Yukari remains determined to pursue his feelings for Misaki, creating a complex love triangle.

Misaki, who also harbors feelings for Yukari, struggles with her own emotions and the societal pressure to comply with the government's system. Ririna, though initially distant, begins to develop genuine affection for Yukari, complicating the dynamics further. The story explores the tension between personal desires and societal expectations, as the characters navigate their relationships under the constraints of the law. Themes of love, duty, and individuality are central, as Yukari, Misaki, and Ririna grapple with their feelings and the consequences of defying or adhering to the system.

The narrative delves into the emotional and psychological struggles of the characters, highlighting their growth and the evolving relationships between them. Secondary characters, such as classmates and family members, add depth to the story, reflecting broader societal attitudes toward the marriage system. The plot unfolds through a series of emotional confrontations, tender moments, and moral dilemmas, as the characters seek to reconcile their personal happiness with the demands of their world. The series balances romance, drama, and social commentary, offering a nuanced exploration of love in a controlled society.