Dragon Ball Z opens with a spacecraft descending into the atmosphere as Goku is heading over to Master Roshi's Island with his son Gohan to visit his friends. The spacecraft lands in the countryside and Raditz emerges out of it shocked that life still exists on the planet. After killing a farmer he flies off to find Goku. Raditz runs into Piccolo who he ridicules and then goes off to confront Goku after his scouter picks Goku up. Raditz finds Goku on Roshi's island and displays his disgust that Raditz hadn't wiped out the human race. Raditz then reveals to Goku that Goku is a Saiyan, that Goku is his brother, and demands Goku rejoin the Saiyan forces. When Goku refuses Raditz kidnaps Gohan and demands that Goku kill 100 people before Gohan is returned. Shortly after Raditz leaves Piccolo shows up and offers to ally with Goku to fight Raditz.

Goku grabs the Dragon Ball Radar and flies off with Piccolo to go track down Raditz. Raditz stuffs Gohan in his pod and is baffled at how Gohan has an unusually high powerlevel, his scouter also shows other high powerlevels around him so he assumes that it's broken. When Goku and Piccolo drop in on him he's somewhat shocked, moreso at the fact at how strong Gohan is than actually seeing the duo ready to fight him. After belittling their strength Raditz catches them both off guard with a hit. Goku and Piccolo take their weighted clothing off and attack Raditz and Raditz easily outmaneuvers both of them and launches blasts at them. Goku avoids Raditz' blast and gets kicked away while Piccolo isn't so lucky, as one of his arms is blown off. This leads Goku into going one on one with Raditz as Piccolo powers up to attempt a new technique he's mastered.

Goku goes on the offensive and attacks Raditz to stall him long enough for Piccolo to power up. Although Raditz toys with Goku and starts to beat him to a pulp Goku is able to hold out long enough for Piccolo to gather enough energy for his attack. Raditz realizes this at the last second and is able to evade Piccolo's attack on him. Afterwards Goku exploits Raditz' tail as a weakness to immobilize him but lets him go before Piccolo finishes powering up a second time. After Raditz starts stomping Goku into the ground again an enraged Gohan breaks out of his pod. Gohan powers up and charges into Raditz, headbutting him and injuring him. Raditz punches him out of the way and Goku once again gets Raditz into a submission to immobilize him. This time not wavering, Piccolo launches his special beam cannon attack mortally wounding both Goku and Raditz. Before Raditz dies he warns Piccolo and Goku that two more Saiyans will be arriving at Earth in a year, and they're much stronger than he is.

Goku succumbs to his injuries and dies but his body is immediately teleported by Kami as he wants Goku to be sent to King Kai's to train in preparation for the arrival of the saiyans. Goku is sent on his way along snake road to get to King Kai's planet. On his way over he runs into some trouble, including falling off of snake way into hell but he manages to get back on his way after completing challenges. Piccolo has taken Gohan after Raditz and Goku's deaths in order to do training after witnessing Gohan's potential power during the ordeal. He leaves Gohan in the wilderness to fend for himself while he does meditative training. After Gohan becomes independant and cunning enough to live and fight on his own Piccolo steps in and the two begin intensively sparring on a daily basis. Tien, Krillin, Yamcha, and Choutzu are brought to Kami's Lookout to begin training of their own for this new threat. They are sent into the hyperbolic time chamber to fight saiyans from the past, face their fears, and spar in order to train. Finally, when Goku makes it to King Kai's planet he is instructed in the art of Kaio Ken and the ability to make the spirit bomb.

Finally a year after the encounter with Raditz two more saiyans (being Nappa and Vegeta) arrive on Earth looking for the Dragonballs. Nappa destroys a city before they move on to confront Earth's special forces. Goku's already been wished back to life and is fast on his way as the fight for Earth begins. Five Saibamen are spawned to fight the Z Fighters and when one self destructs and kills Yamch, Krillin goes crazy and blows the other four away with a huge energy blast. Nappa steps up to the plate to fight after being warned to leave Piccolo alive (as the dragon balls dissapear without the namek) and easily showcases that he overpowers all of the fighters. Nappa individually assaults the Z Fighters and attacks them as a whole relentlessly. He chops one of Tien's hands and part of his forearm off during his attack. Chaotzu plants himself on Nappa's back and self destructs but it is in vain as Nappa is barely phased. Grieved over the loss Tien shoots a beam at Nappa with all of his life force and dies, it also doing very little to Nappa. Vegeta calls a temporary halt to the fight after hearing of the eventual arrival of Goku.

Vegeta gives Goku three hours to arrive as he believes Goku is Kakarot and is interested in seeing how strong he has become since defeating Raditz. Nappa destroys the Navy, Air force, and a town during the intermission and the fighting is resumed after three hours since Goku hasn't shown up. Gohan lands a good hit on Nappa, enraging him and Nappa retaliates by firing a large blast at Gohan. Piccolo jumps in the way of the projectile and sacrifices himself to save Gohan. Shortly after Piccolo dies Goku arrives to take up the fight with Nappa. Goku powers up to show that he has tremendously improved over the past year (much to Vegeta's surprise). Nappa doesn't care and charges Goku and Goku makes very quick work of him. In disgust, Vegeta kills Nappa and then resumes the fight with Goku himself.

Goku and Vegeta battle in which Vegeta is clearly stronger, even outmaneuvering Goku under the effects of Kaio Ken x2. Goku has to push to Kaio Ken x3 in order to overpower Vegeta, which easily does more damage to his body than Vegeta does. After Goku bests Vegeta, Vegeta tries to destroy the Earth but is once again overpowered by Goku. Vegeta then uses a technique to create an artificial moon to transform to his great ape state to kill Goku. After getting smacked around and dodging Vegeta, Goku tries to summon a spirit bomb and is partially unsuccessful. Yajirobe ends up cutting Vegeta's tail off to return him to normal and Goku gives Krillin the power he had summoned for a spirit bomb that hadn't been lost after Vegeta attacked him again. Krillin hits Vegeta with it but it still isn't enough, and unfortunately for Vegeta Gohan's tail has grown back and he transforms into a great ape himself. After hitting Vegeta a few times Vegeta manages to cut off Gohan's tail, but is unable to get out from under him before great ape Gohan crushes Vegeta into the Earth with all of his weight. Vegeta is severely injured and Goku telepathically tells Krillin to show mercy so Vegeta is not killed. Vegeta vows to have his revenge and leaves the planet to go heal. Since Piccolo has died the dragon balls are inert and Mr. Popo tells the remaining Z Fighters (Gohan and Krillin) that they must travel to Planet Namek in order to gather the dragon balls there and revive everyone who was killed in the battle against Vegeta and Nappa.