27-year-old office worker Daikichi Kawachi (Kenichi Matsuyama) rushed back to his hometown to attend his grandfather's funeral, and unexpectedly met his grandfather's illegitimate daughter, the beautiful and pitiful Rin Kaga (Aina Ashida). Rin naturally could not be recognized by the Kaga family, but she attracted Daikichi's attention and sympathy. In the end, Daikichi took this 6-year-old aunt who was still wetting the bed back to Tokyo to raise her alone. Rin's arrival completely changed the life of the bachelor Daikichi. He worked hard in the company for half a day, and when he returned home, he had to take care of Rin's food, clothing and daily life. As Rin reached the age of entering kindergarten, running gradually became the theme of Daikichi's life. In this process, Rin, who had once tightly closed her heart, gradually opened her heart to Daikichi. They were both strange father and daughter, and aunt and nephew with wrong ages. Their ordinary life was full of happiness... This film is adapted from the popular manga of the same name by Yumi Urita.