High school sophomore Yūki Hase noticed that Kaori Fujimiya, his classmate, was always alone. He tried to start a conversation with her but was turned down: "My memories of friendship can last only a week," she said. Despite of knowing this shocking truth, Yūki keeps trying to be Kaori's "new" friend every week.
Hase notices that Kaori seems very detached from the rest of her classmates. Making a daring move, he asks to be her friend, only to find out that at the beginning of every week she forgets everything, and everyone, from the week before.
A new week means a new introduction between Hase and Fujimiya. After they get to know each other again, Hase suggests to Fujimiya that she should keep a diary.
Hase convinces his friend Shogo to eat lunch with him and Fujimiya. Ever the pessimist, Shogo doubts Fujimiya's claim of her memory loss.
Hase begins to grow increasingly jealous of Kiryu and Fujimiya's relationship. After getting in their very first fight, Fujimiya's loses something dear to her while on her way home.
Yamagishi Saki inserts herself into Fujimiya's circle of trust. Together, they experience the joyous moments of friendship others take for granted. Hase has his doubts about the new girl, and trails them as they go out on a "date".
Fujimiya brings her new friends over for a study session at her home. Fujimiya's mother dotes on them all, as all mothers do, excited to see her daughter with friends. Later, Hase meets Fujimiya's mother at the park, and she reveals what she knows about her daughter's memory issues.
While Fujimiya continues making new friends, Hase begins to miss the times when it was just the two of them. With summer break coming up soon, Hase realizes that they wont be able to meet regularly anymore.
Fujimiya and her growing circle of friends take a trip out to the beach, where Shogo helps Hase discover his true feelings for Fujimiya.
The four friends get together on the last day of Summer break to finish up assignments Yuki learns about how much of a difference he's made in Kaori's life.
Hajime has essentially undone all of Yuki's progress with Kaori after some less than savoury facto come to light. Is the girl he once knew gone forever?
Yuki, still interested in being friends with Kaori, sets out to find out why Kaori grew upset with her.
It becomes apparent to Yuki why Kaori is losing her memory so he decides it best to keep his distance from her in order to preserve her memory.