女子高中生速川唯（黑岛结菜 饰）缺乏干劲，上课睡觉，对找男朋友没兴趣，而且是个大胃王，唯一的优点是跑得快。弟弟小尊（下田翔大 饰）与她相反，是个文弱的天才学生，他将仓库改装成研究室，发明了能够穿梭时空的机器。小唯无意中启动了时光机，穿越到战国，以“唯之助”的身份生存下去。她偶遇黑羽城的少主羽木九八郎忠清（健太郎 饰）并爱上了他。小唯决心以小士卒的身份伴其左右，改变少主死亡的历史命运。

In a small fishing village, a gloomy middle school student named Kai meets a mermaid named Lu.

Yuri Kashiwaga was once known as a genius pianist. One day, she returns to her hometown in Goto Islands from Tokyo. There, she begins to work as a temporary teacher at a middle school and advisor for the school chorus. The chorus aims to take part at a competition. Yuri gives the members of the chorus members an assignment. She instructs them to write a letter with the title of “To myself 15 years later.” The 15-year-old boys and girls write about their secrets and worries.