做事得過且過，活在漫天陰霾下的地產經紀 - 志行 (#胡子彤 飾)，從小受母親呵護，一直過著平凡人生。一天，志行重遇小學恩師 - 體育老師黃sir (#譚耀文 飾)。黃sir 為了承諾離世妻子 (陳逸寧 飾)的10公里跑約定，即使患病也要練習參賽。同時，黃sir學生肥妹天心(#蔡裕彤 飾)，為了跟偶像一起跑步，不惜減肥，把生命豁出去，誓要完成人生首個5公里街跑。志行見黃sir、天心各自為著目標奮發，自己卻站在人生交叉點迷路，沒充氣的人生，繼續一分一秒地洩氣。直至某天，一份20年前的回憶忽然來到志行跟前，開始照亮他心裡的陰霾. . .

Chi is a lousy real estate agent who muddles through life. Growing up, his mother has always told him to lead a simple life, until he reunites with his primary school P.E. teacher, Mr Wong.

Mr Wong, who’s been increasingly ill, wants to keep his promise to his late wife to finish a 10k race; At the same time, his student, an overweight student called Tin Sum aims to finish her first ever 5k race just so she can run with her idol. As Mr Wong and Tin Sum fight for their goals, Chi, discouraged, stands at a crossroad and roams aimlessly in life.

Until one day, memories from 20 years ago light up Chi’s world. For once in his life, Chi swears to cheer himself on.