Son Goku has returned from his two years in space, to whoops and cheers from his friends and son. Bulma explains that the mysterious kid told them where he'd be, and asks if he knows him...
But after a greeting to everyone, the strange Super Saiyan asks to speak to Goku alone, away from the group. Goku thanks him for taking care of Freeza, lamenting that he went too easy on him, on Namek. Goku goes onto to say it wasn't needed, as he could have used his new technique, Instantaneous Movement, which he learned on the planet Yardrat, to instantaneously move from his space pod to the battle field. But instead of doing that, Goku decided to be a dick and risk everyone on a mysterious ki he didn't know. (Logical.) The boy curses that he changed history for no reason...
He explains to Goku that he's from the future, 20 years in fact, and requires for assistance; But first, he has to see if Goku can truly control his Super Saiyan state. Goku explains he couldn't at first, but with training...
He got that shit.
The boy follows suit.
(Vegeta's internal monologue: "AHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHHH!")
Suddenly, the boy strikes out at Goku, but our hero doesn't even flitch. He can tell this boy doesn't have the ki of a killer.
After a quick sparring match, the boy makes up his mind. He reveals everything to Goku; His name is Trunks, and in his time, everyone is dead except for him. Dr. Gero, the last surviving member of the Red Ribbon Army (who has never so much as been mentioned off-hand before) created two Artificial Humans (or Cyborgs - but not Androids. Ever.) who will arrive in 3 years time and wreak havoc on the Earth. In that time, they will not only destroy Dr. Gero himself, but kill Vegeta, Piccolo, Tenshinhan, Chaozu, Yamcha, and Kuririn. Gohan will escape and train Trunks, but even he will eventually fall to the Artificial Humans wrath.
But Goku will die before then, of a unique viral infection of his heart.
"Aw, crud, I'm gonna die? What a rip-off! I wanted to fight them!"
Trunks is both surprised and delighted that Goku wants to fight them so badly, and thinks he will be able to save the future. So he gives Goku the medicine for his heart virus, and then reveals who his parents are.
Bulma and Vegeta.
("That seems sort of hamfisted.")
Goku is shocked that Bulma never ended up with Yamcha, and Trunks explains that Yamcha is a playboy and Bulma was tired of his shit.
Think about that for a minute.
The guy who has a panic attack whenever he's close to a woman...
And spends all his time at Bulma's house...
... or at Muten Roshi's house ...
... is a playboy. This guy.
This fucking guy.
Goodbye, Yamucha. We knew ye well.
Trunks leaves for his time machine, and states that he will try his best to meet them in three years time to aid in the defeat of the Artificial Humans. Goku makes his way back to the others, but has a tough time explaining the situation (apparently, future Gohan and future Bulma never told Trunks that Goku isn't good with speaking the mouth words.) - so Piccolo takes over. That's right, Namekian Batman can hear you... even right now.
"SWEAR TO ME."
Skillfully leaving out Trunks' origins, to prevent his existence from being in jeopardy, Piccolo explains the Artificial Humans to everybody. Cue the filler training arc.