Shuichi Kuze’s been spending time with Mizuki Hayama and as they’ve been getting to know each other, they’ve been growing closer. Sadly, Kuze discovers that he’s contracted a fatal illness and decides to break all romantic ties for the little time left to him. Before he can break things off, however, Mizuki tells him that she loves him. Will her declaration change Kuze’s mind? In another city, in the not so distant past, Yuu Himura meets a girl who seems to know him, but he doesn't know her. Soon enough, however, he remembers Yuko Amamiya, a girl he met long ago, and begins to rediscover the depth of their feelings for each other. Will Yuko’s love reach across time to find her true love once and for all?
A flashback reveals Yu and Yuko as high school students who reunite after being apart for ten years. In the present, Muzuki is a guest at Renji's house where she meets Kuze and begins to discover why she was warned to stay away from him when they spend the day together.
Yuko tells Yu that he was her first love and that her feelings for him haven't changed. Mizuki and Kuze continue to spend time together, and she assists him when he collapses unexpectedly. Later, she confesses her love for him.
Yu is concerned that Yuko is being bullied and asks Kuze to keep an eye on her. In the present, Mizuki learns a shocking secret about Kuze, one that will alter the course of their relationship.
Yu and Yuko go shopping together, which she mistakes for a date. Nagi is unhappy with Yu when she learns of the outing. Mizuki and Kuze spend the day together in the countryside where he delivers devastating news.
Yuko continues to be bullied but starts to fight back. Kei and Chihiro reunite and begin to repair the afflictions of the past. Mizuki wants to be strong like Kei and Chihiro and seeks their guidance on how.
Yu finally accepts his love for Yuko, but he has to turn away Nagi. Yu proclaims his devotion and love to Yuko. She reveals a dark secret from her past and implies that he could have saved her.
Yu confronts Yuko's brother with a knife, but he backs down and is beaten up. Mizuki seeks advice from Kei. Kuze treats Mizuki coldly and, later, suffers chest pains that leave him struggling to find the will to survive.
In the past, Yu and Yuko run away together. Yu is out late after an encounter with Yuko's brother. When Yuko goes to look for him, she finds her brother instead. In the present, Yu and Nagi try to reason with Kuze, but they are angered by his response.
Yuko and Yu find themselves hopelessly in love with each other. Yuko's brother assaults Yu when he steps between them during a tirade. Mizuki receives charms from Kei and Chihiro. Kuze finds himself alone as he realizes he wants to live.
Yuko and Yu live together again. Yuko befriends Miki, an orphan with an odd and sad past. Tragedy unfolds on Christmas.
Mizuki sends Kuze a letter, which makes him come out of his house to meet her. She again professes her love to him, which he reciprocates just before his heart gives out. He decides to have surgery in an attempt to live longer.
Everyone spends Christmas with their loved ones. Kuze and Mizuki are living together happily. Yu heads to Japan and meets Yuko in the church. They spend the night together before they say their final goodbyes.