Dragon Ball Z Kai Season 3 Episode 5

Welcome Back, Goku! Confessions of the Mysterious Youth, Trunks!

After three hours of waiting, a space pod crash lands on Earth and Goku emerges from it. There is no time for celebration, however, as the mysterious teenager pulls Goku aside to talk with him in private. After testing Goku's Super Saiyan powers, the teenager reveals that his name is Trunks and that he comes from an apocalyptic future 20 years from the present, as well as that he is the future son of Bulma and Vegeta. He further informs Goku that in three years from now, a pair of deadly androids, built by Doctor Gero of the former Red Ribbon Army, will begin a rampage on Earth and kill all of the Z Fighters, except for Goku, who will have died from a heart virus six months before their assault. Trunks then supplies Goku with a heart antidote made in his future timeline, instructing him to take it when the virus attacks him. Piccolo hears the whole conversation using his special hearing. Goku promises that he and the other Z Fighters will train hard for the next three years to prepare for the androids' assault, and Trunks then returns to the future in his time machine.