Isshuukan Friends.

Synopsis

A rumor begins spreading around the class that Kaori and Shogo are friends. Finding himself a bit envious, Yuki upsets Kaori by asking her to tone down on inquiries about Shogo. However his words hit Kaori harder than expected, resulting in their first fight to which she leaves school early that Friday. Kaori eventually finds herself at the river where she accidentally loses her diary, while at the same time Yuki goes to Kiryu for solace. Kaori then collapses from a cold when she returns home, coincidentally knocking the reminder about her diary off her door. The following Tuesday, Kaori finally shows up for school and Yuki immediately attempts to apologize for their fight. However much to his shock, she fails to remember him or her diary. Feeling depressed, Yuki asks for Shogo's advice and his words encourage Yuki to begin searching for Kaori's lost diary. Afterwards Shogo speaks with Kaori and helps her realize that her memories are missing. The next day, Kaori visits the river on a whim and finds Yuki desperately searching for the thing she forgot. When Yuki eventually finds the diary, Kaori accepts that it was the important thing she had lost. Finally, the pair make up and restart their cycle all over again.

(Source: Wikipedia)