Quick summary(I'll asume that you've already seen up to the point where everyone finishes training to repel the Saya-jin invasion) :
The Saya-jins land in the middle of a city. As soon as Napa emerges from his pod, he blows up the city around them. By their scouters, they determine that there are about six people on the planet with a PL of one thousand, or just over. They zero in on the two highest(Piccolo and Gohan), reasoning that they would be the ones who killed Radiz, and fly off.
Meanwhile, everyone has sensed the arival/explosion and is flying to join up with Piccolo. All the good guys arive, as do the Saya-jins. Napa wants to see a show, so he insists they fight the Saiba Men. Since the Z senshi needed to buy time for Goku to arive, they agree. Tenshinhan beats his, Yamcha is killed by his, Kuririn blows up all the remaining ones.
Napa finally attacks and completly overwhelms everyone. Choazu self destructs trying to kill him. Tenshinhan looses his arm, then dies because he over-did a blast. Piccolo is shot, and dies. Kuririn is just about out cold, and Napa is just about to stepo on Gohan.
Goku arives and slaps Napa around, then uses the Kaioken to take him out. Vegeta kills Napa for being a failure. Goku tells Gohan & Kuririn to go home where it safe, then he and Vegeta go off to fight. Vegeta quickly gets the best of Goku. He out matches the Kaioken and Kaiojen x 2. Goku uses a Kaioken x 3 to slap around Vegeta as easily as he did Napa. The two trade blows, then both launch their highest power energy wave. After upgrading to a Kaioken x 4, Goku sends Vegeta flying.
Vegeta recovers, creates an artifical moon, and turns Ozaru. He then cripples Goku, and is aboiut to kill him as Gohan and Kuririn return. Yajarobi cuts off Vegeta's tail, but Vegeta's normal form is still enough to kick the crap out of the humans. Gohan puts forth a good effort, but gets pounded into the ground.
Gohan's tail grows back, and he goes Ozaru after seeing Vegeta's fake moon. Gohan esentially beats the stuffing out of Vegeta. Vegeta cuts off Gohan's tail, but Gohan falls on Vegeta, crushing him.
Vegeta radios his pod to escape. Kuririn is about to kill Vegeta with Yajerobi's sword, but Goku tells him not to.
Vegeta escapes.