When Mayu Taneda was a child, she met Takuma Kakinouchi, a boy her age who was a patient at the hospital where her father worked as a cardiologist. He became her first love, and the two made a childhood promise to marry each other when they grew up. But their innocent happiness is shadowed by a devastating truth: Takuma has a serious heart condition, and it is expected that he will not live past the age of twenty. Mayu is already aware of his illness, even as Takuma tries to hide the severity of it to spare her feelings.

Years pass, and the story follows Mayu and Takuma as teenagers. Mayu remains deeply devoted to Takuma, her feelings unwavering despite the countdown hanging over his life. However, Takuma, acutely aware that his time is limited and that he may not survive to fulfill their promise, begins to push Mayu away. He distances himself, believing it is the only way to protect her from the greater pain of losing him in the future. This central conflict drives the emotional core of the narrative, as the two struggle between their love for each other and the harsh reality of his condition.

The situation becomes more complicated with the introduction of others who care for them. Mayu finds herself the object of affection for another young man, while Takuma meets Teru Uehara, a female patient who shares his same illness and develops feelings for him. These encounters create misunderstandings and test the strength of Mayu and Takuma's bond. The story explores their jealousy, frustration, and the difficulty of communicating their true feelings when every moment together is precious. Ultimately, the couple must navigate these trials, reaffirming their commitment to each other in the face of Takuma's deteriorating health.

The drama is set in contemporary Japan, moving between the ordinary settings of school and the stark, sterile environment of the hospital, which serves as a constant reminder of Takuma's fate. The narrative arc follows the couple from their innocent childhood meeting through the turmoil of adolescence, leading to the inevitable, heartbreaking climax of Takuma's condition. The live-action series explores themes of first love, sacrifice, and living with the knowledge of an impending loss.