After a year of hard training on everyone's part, the two Saiyans, Vegeta and Nappa, finally arrive. Goku, trying to get every bit of training he can at King Kai's lookout, forgets about how long it will take to travel back to Earth, and cannot be there to confront the two saiyans. So it is up to Piccolo, Tien, Yamcha, Gohan, and Chauzu to hold them off until Goku gets there.
When the battle starts, the Saiyans summon green "Cybamen" to fight for them. The Z warriors manage to kill all of the Cybamen but Yamcha dies when one does a suicide attack. Next Nappa attacks and quickly overpowers everyone killing everybody except for Gohan and Krillin. Goku arrives just in time to save the two remaining Z warriors, and makes short work of Nappa, spinning him over his head. Vegeta finishes off Nappa because he is discraced with his fighting against Goku.
Next Vegeta and Goku fight for a while, and it is pretty evenly matched. Goku uses his Kaoken technique to gain the upper hand and starts hurting Vegeta pretty bad. Vegeta gets mad and launches an orb into the sky, which acts as a full moon, transforming him into a giant monkey called an "Oozaru". Vegeta almost managed to kill Goku when Yajirobe, of all people, jumps in to save the day by cutting Vegeta's tail off.
With Vegeta's tail gone, he turns back into his regular form. Gohan, still having his tail, transforms into an Oozaru, and almost kills Vegeta before his tail gets cut off. Vegeta badly wounded, excapes into his space pod, and launches back into outerspace.