After being dumped by his girlfriend Mami Nanami following a month-long relationship, emotionally devastated college student Kazuya Kinoshita impulsively signs up for a mobile app called Diamond that allows users to rent a girlfriend. He is matched with Chizuru Mizuhara, an exceptionally beautiful and charming young woman who performs the role of the perfect girlfriend on their date. Dissatisfied with what he perceives as inauthenticity, Kazuya gives Chizuru a low rating, which provokes her to confront him about it on their next rental outing. In the middle of that encounter, Kazuya receives a call that his grandmother has collapsed and been hospitalized. Chizuru accompanies him to the hospital, and his grandmother is overjoyed to meet her, believing she is Kazuya’s real girlfriend. Unable to bring himself to shatter his grandmother’s happiness, Kazuya introduces Chizuru as his genuine partner, and she agrees to maintain the charade for the sake of his family.

The situation becomes far more complicated when Kazuya and Chizuru discover they are next-door neighbors in the same apartment building and also attend the same university. At school, Chizuru keeps her rental work hidden by adopting a different appearance, wearing large glasses, tying her hair back, and using her grandmother’s surname, Ichinose. Kazuya continues to rent Chizuru repeatedly to keep up the facade with his family and friends, but genuine feelings begin to develop beneath the transactional arrangement. Meanwhile, Mami reenters the picture and grows suspicious of the relationship, showing signs of jealousy and a desire to reclaim Kazuya for herself. She begins to investigate and subtly work against the fake couple.

The romantic entanglement deepens with the arrival of Ruka Sarashina, a vibrant and assertive girl who initially appears as the rental girlfriend of Kazuya’s friend Shun Kuribayashi. Ruka suffers from a chronically low heart rate, but her pulse spikes to ninety beats per minute for the first time when she encounters Kazuya. Interpreting this as a romantic sign, she declares herself his girlfriend and aggressively pursues a real relationship with him, even using her knowledge of the rental arrangement as leverage. Later, Sumi Sakurasawa is introduced, a painfully shy and timid first-year student who also works as a rental girlfriend for Diamond. At Chizuru’s encouragement, Kazuya goes on practice dates with Sumi to help her gain confidence, and she gradually develops her own feelings for him.

A significant narrative arc revolves around Chizuru’s hidden aspiration of becoming an actress. She took the rental girlfriend job primarily to fund her acting lessons and gain life experience for her craft. When her beloved grandmother Sayuri falls critically ill and later passes away, Chizuru falls into a deep state of grief that she tries to hide. Kazuya is the only person who recognizes her pain and helps pull her out of her depression. Determined to support Chizuru’s dream, Kazuya launches a crowdfunding campaign to produce an independent film featuring her as the lead actress. A new neighbor, the energetic and internet-savvy Mini Yaemori, moves into the building and becomes a key ally, helping Kazuya with the crowdfunding project and encouraging him to express his true feelings. The filming process draws Kazuya and Chizuru closer together, culminating in a trip alone to a hot spring location for the final scene, where the boundary between their rental arrangement and genuine intimacy becomes increasingly blurred. The story continues to explore the tensions among the four main girls—Chizuru, Mami, Ruka, and Sumi—as they each navigate their own complex emotions toward Kazuya, who grows from a directionless and insecure young man into someone capable of deep dedication and sacrifice for the person he loves.