Based on

Movies like Dragon Ball Z: The World's Strongest (1990)

The evil Dr. Kochin uses the dragon balls to resurrect his mentor, Dr. Wheelo, in an effort to take over the world. Dr. Wheelo, his body having been destroyed by the avalanche that killed him fifty years before, desires the body of the strongest fighter in the world as his new vessel. Believing Roshi to be the world's strongest warrior, Dr. Kochin abducts Bulma and forces Roshi to surrender himself to save her. When Goku hears of their abduction, he goes to their rescue.