Dragon Ball Z: Dead Zone

During the events preceding the series, Piccolo trains at a barren cliff but is defeated by a group of enemies. These same enemies then target Gohan, stealing a Dragon Ball from Mount Paozu and defeating Gohan's grandfather, Gyumao, and his mother, Chi-Chi. Goku rushes home to find Chi-Chi injured, and decides to use the Dragon Radar to locate the Dragon Balls and track down Gohan and the kidnappers.