Officer Tuba (Samo, of course) isn't much of a cop. About all he can do is play the tuba in the police band. When his colleague, a sergeant (David Chiang) is killed, he makes Tuba promise to take revenge. And Chiang means it. His ghost keeps coming back, to annoy Tuba into keeping his promise.
Tuba is a coward, and resists, but Chiang fights back by making Tuba do embarrassing things, including making him appear an idiot in front of his sweetheart (Joey Wong) and her parents.