"There's just under a year before the two Saiyans arrive on Earth... Each spent their days practicing, with Gohan training under Piccolo, Krillin under Kami, and Goku under King Kai. The time passed them by quickly, and before they knew it, the battle was upon them. At 11:43 in the morning, the Saiyans broke through the Earth's atmosphere."
Nappa and Vegeta arrive on Earth, Vegeta admits Earth is not as bad as he was expecting. Then they set off to find the Dragon Balls and look for the one who killed Raditz. In the meantime Gohan and Piccolo await for the two Saiyans to show up, Krillin joins them in the wait (It's assumed the other Z-fighters join too, as they are among selectable characters) and then they finally arrive.
After not giving them any information about the Dragon Balls, they release the Saibamen to make them. After escaping/defeating them, the gang faces Nappa, who after a battle with the Z-Fighters unleashes his Nappa Cannon, towards Gohan, but Piccolo sacrifices himself instead. After Piccolo's sacrifice, Gohan fires a Masenko towards Nappa that made his arm feel numb.
Before Nappa can attack Gohan, Goku finally arrives, and battles with Nappa in Gohans place. After defeating Nappa, Goku remarks he can never walk again, and that they should leave Earth. Vegeta, however, betrays and kills Nappa, much to the shock of the others. Goku tells Gohan and Krillin to head for Kame House, before they leave, Krillin asks Goku to switch battle locations, to prevent harming their deceased comrade's bodies more, Goku and Vegeta then fly away to another battle location.