Takuma Kakunouchi Masaki Okada and Mayu Taneda Mao Inoue begin as childhood friends, their bond forming long before they fully understand the weight of Takuma’s heart condition. A cardiologist, who happens to be Mayu’s father, treats him as Takuma learns that his life may end far earlier than anyone hopes. The two friends overhear a stark truth: Takuma might not live past twenty because of his illness. Yet this grim knowledge does not dampen their closeness; instead, it deepens their connection and blossoms into a first love. In a field full of possibilities, Takuma promises to marry Mayu when they both turn twenty, sealing a dream that will test the limits of time and heart alike.
As they move into junior high, their feelings endure even as life grows more complicated. Takuma, burdened by the knowledge of his limited years, tries to distance himself to spare Mayu further heartbreak, believing that he must leave her behind after his final school day. He sets his sights on an elite high school, convinced that Mayu can’t enter the same prestigious institution. To his surprise, Mayu not only enters but excels enough to become a first-year student body representative. Her presence disrupts Takuma’s resolve: she openly challenges him in the school hall, reminding him that she won’t be pushed away, and soon the community recognizes them as a couple.
At school, Mayu’s path crosses with Kou Suzuya [Yoshihiko Hosoda], who asks her to be his girlfriend, but she declines. Meanwhile, Takuma reconnects with Teru Uehara [Natsuki Harada], another patient with the same heart condition, and Mayu feels a twinge of jealousy as Takuma spends more time with her. Kou even suggests that Takuma should hand Mayu over to him to spare her from further pain. The pair briefly break up, wound by circumstance, until Takuma honors Teru’s wish to share a kiss before she dies. The very next day, Teru passes away.
A public challenge follows: a 100-meter race proposed by Kou. Takuma wins, and afterward he shortly seeks a date with Mayu. But fate intervenes when Kou, leaving the school at the same time, is struck by an oncoming train. In the aftermath, Takuma learns there may be a donor heart available.
Mayu discovers the donor is Kou, now brain-dead, and she hides this fact from Takuma, hoping to spare him further heartbreak. Takuma eventually learns the truth and refuses the donation, while Kou’s family—moved by signs of emotion they’d seen in him—initially refuse to allow the transplant, despite Kou having registered as a donor.
Takuma suffers a heart attack and, in the hospital, doctors acknowledge he has a slim chance of recovery. In a quiet moment of faith, he wishes for even a little more time to live and miraculously pulls through. He and Mayu embark on what they call a “honeymoon,” visiting places that hold meaning for their shared past. Along the way, Takuma gives Mayu his handwritten will, written when they were eight, as a symbol of his enduring love.
Back at the hospital, another heart attack proves fatal despite the best efforts of the medical team. Mayu climbs to the rooftop and reads his letter, which urges everyone to be happy even after he’s gone. In the final act, she borrows his urn and goes to a church for a symbolic “wedding,” completing the promise they made long ago. A closing flashback returns to the day they first met in the hospital when they were eight, underscoring the film’s meditation on love, memory, and the fragile thread that holds life together.
