He is one of the last four purebred Saiyans (Nappa, Vegeta, Goku and Raditz). He is sent to see if his brother has destroyed all life on the planet of earth (so it can be sold to another race), but to his surprise it wasn’t (because when Goku was young he fell off a mountain and hit his head on a rock which made him forget about his mission), so Raditz kidnaps Gohan and tells Goku to kill 100 people or his son will get killed, so instead of killing the people he allies with Piccolo and so Piccolo eventually defeats Raditz by using his Special Beam Cannon attack (but the only catch was Goku had to hold Raditz to be able to use it, so Goku died also).