Memories Off Sousou: Not Always True

Kusumegi Itsuki, a second-year student at Fujikawa High School attached to Keiyo University, has a fiancee whose face and name he does not know.
Because of her presence, Itsuki has never been interested in romance.

His 17th birthday, when he would find out who his fiancee was, was approaching in about a month.
One day, in order to hold a meeting to comfort a classmate who had lost her heart, Itsuki goes to Fujikawa's family restaurant "Russac'', but ends up getting into trouble with a part-time female clerk there.
Memories Off Sousou: Not Always True - Play-Asia.com
The clerk, Sunomiya Sae, was a third-year senior at the same high school.
Although the way they met was the worst, Itsuki soon began to admire and respect Sae after seeing her mature demeanor.

His childhood friend Amaha Nene sees Itsuki and begins supporting his relationship with Sae with all her might.
The season is autumn. Itsuki's peaceful high school life is about to change little by little.