The Saiyan Saga starts off Dragon Ball Z. Its when Goku is older and we his see first son, Gohan, for the first time.
We start off by seeing an alien space ship. It lands near a farm. The farmer decides to figure out what it is. He thinks it could just be a meteor, then he finds its made of metal. All of a sudden a hatch opens and a hand reaches out. A figure comes out of the space pod and slowly hovers out of the crater. Seeing the farmer hes confused on why Kakorott hasnt destroyed this planet. Who's this Kakorott? Anyways, the farmer aims a shot gun at the figure, and fires. In an instant the bullet reaches its target, but the figure easily catches the bullet. By now the farmer is scared to death. The figure flings the bullet at the farmer, killing the farmer. We then see some strange computer on the alien's face indicating a power level. We then see him flying off.
We cut to Goku getting fire wood, and hauling an entire tree off. Chichi, Goku's wife, is concerned about Gohan because she cant find him. Goku says he'll find him, with that hes off on the Flying Nimbus. We then see Gohan, hanging from a branch about to fall off a cliff. Hes crying his head off, as Goku slowly hovers down. Goku puts him on the Nimbus and theyre off for a reunion with their friends at Kame Island.
We then see a figure. It is Piccolo, Goku's number one enemy, or so he thought. All of a sudden Piccolo senses an enormous power level. In the distance we see the alien that killed the farmer. The alien makes Piccolo mad so Piccolo fires a blast at him. When the smoke clears we see the alien unscratched claiming Piccolo had only managed to singe some of his leg hairs. The alien is about to show Piccolo his "Keep You Eye On The Birdie, when he picks something up on his eye computer, or scouter. We then see him flying off.
At the Kame House we see Master Roshi and Krillin preparing for the reunion. Bulma then walks in wandering where Goku is. We then see Goku and Gohan flying on the Flying Nimbus towards the Kame House. They land and meet up with everyone. This is when Krillin, Master Roshi, and Bulma are surprised to see Goku has a son.......WITH A TAIL! Goku explains about how Chichi is over protective of Gohan.
In the middle of the conversation, Goku senses an enourmous evil power. Soon Krillin senses it too. Suddenly the alien appears. He reconizes Goku immediately. Goku has no idea who this alien is or why he keeps calling Goku Kakorott. The alien then says hes from a race known as the Saiyans. There beings with incredible powers who destroy whole races and then put the planet the race was on for market. Master Roshi then tells Goku about how Goku's grandfather, Gohan Sr., actually found him in a space pod and how at first Goku was mean and hateful. Goku fell into a ravine though and seriously bumped his head. Goku quickly recovered though and became a nice loving boy. Afterwards, the alien smiles and says, "thats right, I'm Raditz, your big brother." Everyone was shocked.
Raditz told that their home planet, Vegeta, was destroyed a few years back and only 4 Saiyans lived. Raditz wants Goku to join him and the other 2 Saiyans, but Goku refuses. Raditz notices Goku has no tail. When asked about it, Goku replies he had it surgically removed for good. Raditz then sees Gohan. Raditz realizes he is a Saiyan because he has a tail. Raditz takes Gohan, and says that if Goku doesnt bring him 100 dead humans in a day, Gohan dies. With that he flies off with Gohan.