During his youth, Itsuki Usawa often took care of Yui Kameyama, 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? (Source: MU)

Usagi to Kame Series was later sold as ebook chapters under the title Osanakatta Kimi kara no Kokuhaku wa, Jikou desu ka? (幼かった君からの告白は、時効ですか?) by Otometika Shuppan from July 13, 2018 to October 19, 2018.