Dragon Ball Z open with Goku going to visit his old friends on Master Roshi's island. But soon Piccolo, Goku's arch nemesis, arrives followed shortly by Goku's older Brother Raditz. Raditz tells Goku that he is a Saiyan from outer space, and that he is to rejoin the Saiyan forces. Goku refuses, and Raditz kidnaps his son, Gohan. Piccolo decides to join forces with Goku temporarily to end this new menace.
The battle ensues with Raditz easily holding off the attacks by Goku and Piccolo. Then finally Piccolo finds a way. His Special Beam Cannon, a move he perfected to use against Goku, finds purchase, wounding Raditz as he barely dodges, but gets nicked in passing. Knowing that it would be the only way, Goku sacrifices himself to hold Raditz still while Piccolo fires one last time. The beam travels through both Raditz and Goku, killing them both.
After Goku's death, Kame, the guardian of the Earth, requests he be sent to King Kai's place for special training. The Earth's defenders find out that more Saiyans, much more powerful than Raditz, will be coming soon. Piccolo takes Gohan, after seeing his high power level, for special training for the upcoming fight. The other fighters, Tien, Yamcha, Krillin, and Choazu, begin training for the Saiyans' arrival. Goku makes it to King Kai's after a couple misadventures on the Snake Way path and begins his training.
One Year after Goku's death the Saiyans arrive on Earth, looking for the dragon balls. The Earth's special forces arrive after Napa destroys a small city. Soon the media and the military arrive to watch over the monumental battle. Vegeta and Napa, the two saiyans, decide to employ the help of their Saibamen, small green men who grow out of a seed planted in the ground in a couple minutes time. Piccolo, Gohan, Krillin, Yamcha, Tien, and Choazu decide to fight the Saibamen one at a time. Vegeta agrees to those terms and the tournament like battle ensues. Yamcha beats a Saibaman, but while his back is turned, it jumps from the ground and latches onto him. It selfdestructs, killing Yamcha in the process. Krillin gets really mad and fires off a blast that starts the end of the Saibamen. Once the team has beaten the Saibamen, the bigger of the two saiyans, Napa, steps up to battle.
No one is even able to touch Napa, who proves way to strong and fast for the outclassed humans. Choazu comes the closest when he attempts to pull Napa into an alternate dimension (kill him), but Napa blocks just in time and Choazu ends up dead. Tien goes into a rage, and launches a suicide attack against the giant alien. Before Napa finally kills Tien, he chops his arm off. Napa is a little bruised after the fight with Tien but he is pretty unharmed. Just as Napa goes to fight against the remaining 3 fighters, Vegeta calls a time out to give Kakorot (Goku) time to arrive on the scene. The 3 hour time out expires and the fighting resumes. Napa launches a blast at Gohan, and Piccolo jumps in front of the blast and takes the shot. Piccolo dies from Napa's blast, but Gohan is spared. Napa's assault continues against Gohan and Krillin. Enter Goku!!
Goku makes sport of Napa for a bit before eventually Vegeta, sick of his partner's incompetence, kills Napa. That leaves Vegeta and Goku to battle it out. The fights runs to a stalemate before Vegeta pulls out his ace, a blast of energy meant to mimic the full moon. Vegeta goes into Oozaru, big monkey, form and starts to convincingly win the fight. Eventually the cowardly Yajirobe comes to the rescue cutting off Vegeta's tail. Goku blasts him with a kamehameha, sending him far far away. Yajirobe leaves before Vegeta can come back, and Goku tries to gather what little energy he has left for the rest of the fight.
After the battle ends and Vegeta retreats back to space, Goku, Krillin, and Gohan are rushed to the hospital. Krillin and Gohan are out in no time but Goku isn't so lucky. He's in a near body cast. Now that Piccolo has died that means Kame has died, due to their link. So Bulma decides to lead an expedition to Kame's home planet, Namek, to use their dragon balls to revive their fallen comrades. They acquire Kame's old ship and take off for the far away planet. They run into a couple obstacles along the way, a fake mockup of Namek with some malicious aliens, and a spaceship filled with refugees who think they are the bad guys. But they eventually arrive on the Namek. Goku gets out of the hospital when Yajirobe brings him a Sensu Bean to heal him. He gets a ship from Bulma's father, Dr. Briefs, and heads toward Namek to help his friends.