"I'm your boyfriend... from the future." High school students Dain Son and Sihyun Lee could hardly be called close. One day, Dain is approached by a man who claims to be Sihyun from the future. What’s more, he says they end up dating and he has traveled back in time to stop a murder. As Dain and “future Sihyun'' become more involved, Dain and “present Sihyun”’s friendship also changes. What will become of such entangled relationships? How will the future change?