Osanakatta Kimi kara no Kokuhaku wa, Jikou desu ka?

During his youth, Usawa Itsuki often took care of Kameyama Yui, Itsuki's neighbor and several years his junior. When she was in elementary school, Yui declared she would become Itsuki's bride. Years pass, and when they meet again, Itsuki turns out to be a teacher at Yui's high school. Will their relationship develop into something more than just neighbors?