國中二年級的阿山與小學三年級的阿嘉是對熱愛棒球的小兄弟，而阿山心儀的女孩 - 佩琳就快要搬家轉學了。就在佩琳搬家的前一天，阿山與佩琳約定隔天要交換彼此最心愛的簽名球...
Shan's crush is going to move away and they make a promise of exchanging their favorite signed baseballs. His signed ball, however, is lost because of his brother Jia, so the brothers start a journey to find the ball...
