Still recovering from her beloved younger sister Haru's death, Natsumi harbors a terrible secret. She is currently dating her dead sister's fiancé, Tougo. Although Haru and Tougo were only to have an arranged marriage of convenience, Natsumi knew that her sister's feelings toward him were utterly genuine. Despite this, when Tougo asked Natsumi if she wanted to become his new girlfriend, she agreed, but on one condition: he must take her to all the places he went with Haru.

Torn with shame, Natsumi meets him every week, seeing the places he visited with Haru and hearing about their experiences. Parting ways after a notably painful encounter overcome with guilt, she is ready to join her sister and begins to move in front of an oncoming train—until Tougo stops her and announces that if she dies, he will too.

As the curse of spring leads to a fleeting summer, two people connected by death find solace in each other, grappling both with their own feelings and the ever-present memory of one long gone.