Mini Mo (Sammi Cheng) is a transplanted Hong Kong resident who makes Japan her home. She falls in love with budding pianist, Kurokawa. Unfortunately, Kurokawa is forced to break up with Mini, having become successful in his skills. This forced parting drives Mini to eat relentlessly and ten years later, she balloons to over three hundred pounds. Kurokawa has become a national treasure, and tours the country with Mini showing up at every concert. However, he does not recognize her, driving Mini into further depression. Following one of Kurokawa's concerts, after seeing Kurokawa and his newly engaged fiance, Mini decides to hang herself, until Fatso (Andy Lau), an equally overweight knife salesman accidentally saves Mini, becoming her friend and confidant. She hangs on to him because she feels she has nothing left, and Fatso takes to her only after he discovers that she is really a sweet girl. On a national radio program, Kurokawa reveals his strong feelings for Mini have not ceased. Before they parted, they made a mutual promise that if the two ever lost track of one another, they would meet ten years in the future at the bottom of the Tokyo Tower. Mini wants to meet him, but she is afraid he will reject her current form and opts for suicide a second time, until Fatso promises to ensure he gets her back into shape and meet her former lover. Through various comedic methods of losing weight, such as swallowing a tapeworm to exercising using Dance Dance Revolution, Mini gradually becomes thinner, but Fatso finds that his finances are running low. He decides to open up a boxing gig on the streets, where any pedestrian can pummel him for a price. Once Fatso earns enough and with Mini looking slimmer, he enrolls her into a weight loss institution. The program proves successful and Mini regains her former appearance. Fatso drops Mini off at the Tokyo Tower and the two say their goodbyes. Kurokawa, with the mass media swarming under the tower, reunites with Mini. The pair instantly become national celebrities and following a television special at a local network, Mini accidentally notices another program broadcasting Fatso's ongoing boxing gig (Mini was unaware that he was doing this to earn for her enrolment in the institution). Taken back by his motivations to make her thin, Mini tearfully apologizes to Kurokawa and leaves him. Months later, Mini has written a new, best-selling book called "Dieting Happy", earning her national fame, yet she is still disappointed at not being able to find Fatso. However, while driving around the city streets, she notices the same car that Fatso drives near a local park. There, Mini finds "Fatso", albeit having lost much weight and carrying her best-selling book, stating he misses her. With a promise never to leave one another again, the two embrace.