Seventeen-year-old Yuichi Ezaki is confined to a hospital in the city of Ise with a case of hepatitis A, spending his days bored and frequently trying to sneak out against the rules. A brusque but caring nurse, Akiko Tanizaki, makes him a deal: if he can befriend a difficult new patient, she will overlook his behavior. That patient is Rika Akiba, a beautiful girl his own age who has spent most of her life in the hospital due to a serious heart valve condition. Their first meeting is a disaster when Yuichi lies about having read her favorite book, but an awkward bond slowly begins to form.

As Yuichi learns the true severity of Rika’s condition, knowing that a dangerous surgery may be her only option, he becomes determined to show her the world outside the hospital walls. Their developing romance faces constant interference, not only from Rika’s own fear and sharp personality but also from the hospital staff. The head physician, Dr. Gorō Natsume, who has a deep, complicated history with Rika and lost his own wife to a similar illness, is hostile to their relationship and violently confronts Yuichi. Meanwhile, Nurse Akiko secretly supports them, understanding what little time Rika might have left.

Across thirteen episodes, the story follows Yuichi and Rika’s journey as they share stolen moments reading, watching sunsets, and attempting small escapes, all while confronting the fragility of life. The narrative also includes the influence of other patients, such as the lecherous but insightful elderly man Tada Yoshizō, who leaves Yuichi an unexpected and embarrassing inheritance. With the help of his loyal friends from school, Yuichi works to fulfill a promise to take Rika to see the half-moon from a place she has always dreamed of visiting, culminating in a desperate and emotional race against her declining health.