After defeating Freeza, Goku returns to Earth and goes on a camping trip with Gohan and Krillin. Everything is normal until Cooler - Freeza's brother - sends three henchmen after Goku. A long fight ensues between our heroes and Cooler, in which he transforms into the fourth stage of his evolution and has the edge in the fight...until Goku transforms into Super Saiyan.

(Source: Anime News Network)