The story begins with Hase Yuuki taking an interest in befriending his classmate and crush Fujimiya Kaori. At first, Fujimiya is shown to be rather cold towards Hase. This is explained later when she reveals that she keeps people at a distance because she forgets the memories of her friends each Monday. After class, Hase helps Fujimiya carry notebooks where he tells Fujimiya that he'd like for them to be friends. Fujimiya says that she is happy and appreciates the thought of him trying to befriend her, but claims that "it is useless for her to make friends".
During lunch, Hase goes to the rooftop and offers to have lunch with Fujimiya. He questions why she isn't able to have friends even though that made her happy. He wonders if it's because her parents are strict, but actually it is because she is worried that when her memory resets the following Monday, that she won't recognize him.
Despite Fujimiya's attempts to keep Hase distant by explicitly stating they are not friends, Hase is determined to meet her during lunch every day on the rooftop. Fujimiya's attempt to keep Hasse distant can be potentially explained as a way to get around losing memory of Hase if she doesn't consider him as a friend. Hase consults with his close friend Kiryuu Shougo about why Fujimiya seems to hate him because she often says that they are not friends. Hase seems to be sensitive to matters concerning Fujimiya since he has feelings for her and her condition to forget her memories make him overprotective of her. Hase and Fujimiya enjoys lunch every day on the roof top for the whole week, and they share a close friendship even though Fujimiya stills says that Hase isn't her friend, which often makes him feel down.
However, the following Monday when they return to school, Fujimiya completely forgets Hase. She finds the act of Hase being friendly towards her strange, since to her Hase is a stranger but to Hase she is his close friend. They meet again during the rooftop and he is able to convice Fujimiya that last week they spent lunch together and that they were friends. Fujimiya believes him because her memory of last week during lunch was blurry, which would be explained by her having a friend at lunch. As a result, they are able to continue their friendship and Fujimiya begins to trust Hase because he showed that he will always try to be her friend.
Hase Suggests that Fujimiya keeps a diary so that she might remember some of the things that happened when rereading her diary. She does this and begins to take detailed notes on what is happening around her, concerning her friends, and especially Hase.
It is important to note that in the manga Fujimiya thinks to herself that she had already tried keeping a diary before Hase suggested it. It didn't end up working well because she couldn't remember the memories written in her diary and she no longer felt the emotions there, which made her sad so she stopped the diary.
The next day she begins to make him lunch because she thinks that's what friends do based on the manga reads and this continues throughout the story. She notes that Hase likes 18 grams of sugar in his egg sandwich, which becomes significant to their friendship later on. The 18th of every month soon becomes the Day of the 18th or (Hase Day in the anime), because its the number that Fujimiya associates with Hase. She is seen to recover some of her lost memories concerning Hase when she wrote 18 on the board as a solution to a math problem.
Next week, Hase and Fujimiya meets on the rooftop once again. However, this time Fujimiya had read her diary before coming to school. Fujimiya tries to come of as already friends with Hase, even though to the current Fujimiya, Hase is a stranger once again because she lost her memories. Hase sees through Fujimiya's attempt immediately, and Hase apologizes profusely for pushing too hard on her and being selfish which caused her to lie saying that she remembers him when she does not. Hase says that he thought about only himself by having Fujimiya keep a diary to save his own trouble of having to do the asking. Hase states that he's an idiot for skipping the most important process, which is asking if they can be friends. Following his apology, Fujimiya is happy and notes down in her diary that Haise thought about himself and made her keep a diary, when he realized his mistake and he immediately apologized, so he really is a good person. Hase asks Fujimiya if they can be friends again and she smiles happily says that "I guess it can't be helped".
Hase and Fujimiya friendship seems to grow closer each week despite Fujimiya memory being "reset" by Monday. Fujimiya views Hase as a special friend. She even agrees to meet with Kiryuu and tell him her story. They have a rough first encounter and Kiryuu is seen as intimidating, which makes him hard to talk to. As a result, to Fujimiya, Kiryuu is strictly speaking not a friend.
This is proven to be advantageous as Fujimiya loses her diary following an argument with Hase. As a result, she doesn't remember Hase next week, but luckily she still remember Kiryuu because he's not her friend. Hase thought Fujimiya might have purposely thrown the diary away because she no longer wanted to be friends with him. But Kiryuu assures him that Fujimiya is not the type of person to throw away something so important to her over a small argument. Hase then desperately begins to search for her diary.
Fujimiya also realizes shes lost something important. Her mom wonders why she stopped making her lunch for her special friend, which sparks her to ask Kiryuu what Hase is looking for in hopes that it's what she lost. Kiryuu is not as straight forward, he questions Fujimiya what it might have been that she lost. And if it was important to her, would she have thrown it away. Before Kiryuu reveals what it is important that she lost, Fujimiya stops him and says that she wants to remember it by herself because she feels that it is important to her. She heads out to look for the thing she lost, and before she leaves Kiryuu tells her that if she bumps into Hase who skipped morning class and has been looking for her journal since, that she should say hi to him because it would make him very happy.
Fujimiya meets Hase near the river where she misplaced her journal. Hase is covered with mud and his hands are bruised from searching for her diary through grass. She calls out to him saying "Hase-kun" which is a surprise because it suggests that at least part of her memory of him has returned. Hase asks her if there was any chance that she might have thrown away something important to her. Fujimiya says that no way would she ever throw something that precious to her. Hase is continues his search in hopes that the important thing Fujimiya is looking for is the same thing that he is looking for, the diary containing her memories detailing her friends. Hase does retrieve diary and he asks her if she want to read it. Fujimiya says that before even needing to read the diary, she can tell the diary was the important thing she was looking for. She thanks Hase, who blushes and says that it's nothing.
After the diary incident, Hase and Fujimiya are even closer than before. To Fujimiya, Hase is a special friend. With the introduction of new friends such as Yamagishi Saki a lethargic girl with bad memory and two girls, Hase and Fujimiya are no longer able to spend time by themself on the rooftop every day. This initially makes Hase a little sad and jealous as he wants to spend time alone with Fujimiya, especially when Fujimiya talked about Kiryuu. Hase is seen to have feelings towards her, which causes him to be somewhat overprotective and irrational, but upon seeing Fujimiya happy with having friends, Hase is happy for her and it shows that he places her feelings before his.
During the summer break, the group of friends head to beach. Unfortunately it was raining so they stayed in the arcades to wait out the rain, where Kiryuu showed his mastery over the arcade machines, After the raining stopped, the friends were able to go back to the beach and Hase and Fujimiya were able to walk alone and chat.
Hase is worried that Fujimiya might lose her memory again and it makes him sad when she forgets him. With the introduction of a transfer student and the childhood friend of Fujimiya, Hajime Kujou, Hase's fear become a reality as Fujimiya completely loses all her memory of Hase. Hase describes the feeling of losing all of the progress he has built up with Fujimiya because of a "jerk", referring to Kujou who calls Fujimiya a traitor for breaking a promise which lead to Fujimiya returning to her cold self before she became friends with Hase.
The group goes on a school trip. During the trip Hase is more hesitant in regards of his approach towards Fujimiya. While still clearly having feelings towards her, Hase keeps her distant and justifies that it's better for them to be just friends and not special friends since his role as her special friend has according to him seemingly faded with each addition of Fujimiya's new friend, such as Hajime, Saki, etc. However, this is not the case for Fujimiya, as she as well as everyone else in the friend circle begins to worry about Hase because he isn't being himself and he looks clearly more sad in the recent weeks which Fujimiya writes about in her diary.
In the backstory, it is explained that Fujimiya loses her memory on the Sunday when she planned to meet Hajime at a park before he moved away. During this time Hajime was Fujimiya's special friend much like what Hase is currently to Fujimiya. She got bullied because she was closer to Hajime than one of her other friends who had feelings for Hajime. Her two close friends at the time went against her and told her that no friend would steal away the make the first move on the love interest of another friend and tells Fujimiya that she is no longer their friend. This triggers her memory of her friends being reset every Sunday, because Fujimiya starts to question the worth of friendship. That at one point people could be seemingly friends and then the next they are no longer friends.
After Fujimiya confronts her past with the help of her friends' support, she comes accept the past. This confrontation allows Fujimiya to remember everything that has happened until that point, including earlier forgotten memories of Hase and Hajime, additionally her memory no longer resets.
In the Epilogue, Hase begins to say that he would like to be move than friends with Fujimiya, instead of just friends like he usually says every Monday. But he was interrupted by Yamagishi and their friends. They briefly tease him asking what he was about to say next. The friends say that they would like to stay friends even though they graduated and are going to different schools. Fujimiya expresses her desire to hang out with her friends and notes specifically that she would like to call Hase every day and be able to sometimes spend time alone with just him.