Dragon Ball Kai • S01E57 : Welcome Back Son Goku! The Confession of the Mysterious Young Boy 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 apocolyptic 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 Dr. 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 die from a heart virus some time before their assault. Trunks then supplies Goku with a heart antidote that Future Bulma made for him, instructing him to take it when the virus attacks him. 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 returns to the future in his time machine.