Dragon Ball Z - episode 20 - 19 Sep, 1989

After Goku completes the first phase of his special martial arts training King Kai instructs him to bash Gregory with a hammer. Easier said than done. After an exhaustive chase, King Kai tells the story of the Saiyan race. Inspired, Goku resumes his training and eventually manages to put the hammer to Gregory. Impressed, King Kai agrees to train Goku personally. But will it be enough to stop the incoming Saiyan invasion of Earth? (Source: dbzepisode.org)