College student Chitose Tada spends his birthday alone, feeling lonely as he wanders through Tokyo. On the night of his solitary birthday on March 30th, he finds himself remembering a promise made by a high school classmate named Koutaka, who had sworn to celebrate Chitoses birthday in a big way. This sets off an extended flashback to Chitoses second year of high school, when the tall, handsome, and athletic Koutaka transferred to his school. Initially put off by Koutakas unsociable and blunt demeanor, Chitose is paired with him as a battery partner on the schools baseball team. Through their time together on the field, Chitose comes to understand Koutakas true nature and finds himself drawn to him. The two boys gradually become aware of each other and begin an awkward but sincere romantic relationship.

Both characters have complicated feelings about birthdays due to past family drama, but as a couple, they discover new ways to make the days special. However, their happiness is disrupted when they begin discussing their future career paths after graduation. This creates a rift between them, and they grow distant. Despite Koutakas earlier promise to celebrate Chitoses birthday, graduation arrives and they lose touch completely. For two years, their message screens remain static, with no communication between them. The story alternates between the present day, showing Chitoses heartbroken and pining state as he wanders through Tokyo, and the happier high school flashbacks that reveal the depth of their relationship. As his lonely birthday night progresses, Chitose finally works up the courage to send a new message to Koutaka, setting the stage for a potential reunion. The manga builds toward a reconciliation featuring a grand gesture that provides a satisfying and emotionally cathartic happy ending.