Future Trunks comes to the present from the year 784 AD. Goku died from a viral heart disease

he picked up on the planet Yardrat and soon three years later, the Artificial Humans Android 17

 and 18 showed up, and killed all of the Z Fighters, except Gohan. As Trunks grows older, Gohan

trains him, but eventually he is killed by the Androids .Then that makes Trunks very mad so he becomes a Super Saiyan. His mother Bulma invents a time machine that will allow Trunks to travel back through time to the year 764 AD and gives Goku the medicine to cure his disease. He also came to warn Goku of the arrival of Android 17 and 18 will be coming in three years from then. Trunks later returns to the past, only to find it turning out different from his future. He stays to fight in order to atone for his sins of changing the future.