Trunks Saga Rulebook
The evil Frieza survived the demise of Planet Namek. He was rebuilt with cybernetic parts making him even more powerful than before. Bent on revenge, he and his father King Cold come to destroy Earth and Goku.
Earth's heroes (except Goku who still has not arrived following the destruction of Planet Namek), assemble to fight Frieza. They know they must stop Frieza now or Earth will be destroyed. But they also know that they are no match for Frieza and cannot possibly win.
Unexpectedly a powerful stranger arrives whom challenges and defeats Frieza and his father King Cold. This stranger knows the exact time and place Goku will arrive and can incredibly go Super Saiyan at will.
When Goku arrives, the stranger reveals who he is and why he came to Earth. His name is Trunks. He is from the future. In his timeline, the Earth is being controlled by the evil Androids. He is the only good guy left and has no hope of defeating the Androids by himself. His last hope has been to travel back in time.
In Trunk's time, Goku had passed away from a heart virus. Trunks has traveled back in time to give him an antidote for the virus so that Goku can survive and help defeat the Androids. His task completed, Trunks goes back to his own timeline while the Z Fighters train for the appearance of the Androids in three years time.