A year and a half after planet Namek's destruction, many of the characters of Dragon Ball Z begin to regain a normal life. They are still waiting for the day when Goku finally returns home. Gohan, who has become worried of when his father will come home, begins to slack off on his studies and his mother hires a private tutor. However, she finds out that tutor is abusive and chases him off after he insults Goku. Meanwhile, Vegeta returns to Earth after failing to find any trace of Goku. Although rude and hostile, he reluctantly accepts Bulma's offer to live at Capsule Corporation since he has no where else to go. Bulma has developed some form of affection for him, but Vegeta does not notice this. Krillin has gotten over his breakup with Maron and resumes his training, as do Piccolo and Tien Shinhan. Unbeknownst to them all, they are once again threatened. This time, Frieza has returned, having survived his fight against Goku on Namek. Frieza has been cybernetically enhanced since much of his body was destroyed when Namek exploded. He is arriving to Earth with his equally diabolical father, King Cold, to exact revenge against Goku and the other Z Fighters for humiliating him by defeating him. Gohan and the others sense Frieza's ki, which is much stronger than when they first fought him. They fly to the Northern Wastelands and wait for him so they can stop the tyrant. Once they gather, Frieza and his father arrive on Earth and Frieza orders his minions to destroy the first Earthlings they find. But before they can, they are killed by a mysterious young fighter, later revealed to be Future Trunks. Future Trunks tells Frieza that he will be the one to destroy him, but Frieza mocks the warrior and asks how he knows Goku. Future Trunks does not say how but says that he will show what a Super Saiyan can truly do, which shocks Frieza that this boy is a Super Saiyan as well but makes him think he is being decieved. Future Trunks transforms into a Super Saiyan and it is clear that his power has overcome Frieza's. He is able to dodge and throw back Frieza's Supernova before slicing him in half and destroying his remains. King Cold, fearful of Future Trunks, asks him to join him as Frieza's replacement. Future Trunks rejects him. King Cold asks for Future Trunks' sword and tries to kill him with it, but Future Trunks effortlessly kills him with two energy blasts and destroys the ship.
The Z Fighters are shocked that Future Trunks killed Frieza so easily and that he is a Super Saiyan. Vegeta wonders how he could be a Saiyan due that he, Goku, and Gohan (being a half-breed) are the last Saiyans alive. He tells them that Goku is due to arrive in 3 hours and they wait for him. They try to ask Future Trunks who he is and where he came from, but Future Trunks refuses to reveal anything about himself other than he is 17-years-old. Bulma points out to Krillin that Future Trunks resembles Vegeta. Vegeta gets irritated at Future Trunks for constantly looking at him. When Goku arrives, Bulma tells him that Future Trunks killed Frieza and his father, and Piccolo says Future Trunks transformed into a Super Saiyan. Goku is amazed that Future Trunks transformed at his young age, but Vegeta reminds him that they and Gohan are the last Saiyans. Future Trunks asks Goku to talk alone with him. He asks Goku to transform for him, which Goku does, and Future Trunks attempts to strike him with his sword. He stops halfway and Goku does not react because he sensed no malice in Future Trunks. Then, Future Trunks tries several more strikes which Goku stops with his finger. Future Trunks concludes that Goku is indeed powerful, and Vegeta is angry that Goku and now a stranger he has never seen before have surpassed him. After swearing Goku to secrecy, Future Trunks introduces himself and that he is from the future 20 years from now. He says that he is actually Half-Saiyan and that Vegeta is his biological father, much to Goku's surprise that Vegeta would have a child, and he will be born 2 years from now. Future Trunks tells him the real reason he came; he warns him that 3 years from now, on May 12 at 10:00 A.M. near South City, two Red Ribbon Androids will appear and destroy everything on Earth. He reveals that he is the only fighter left; all the Z Fighters will be killed in combat and only Gohan escaped before being killed 13 years later. He tells Goku that he will die from a heart virus that has no cure in the present before the battle, but Goku is upset that he will not be able to fight the androids. Future Trunks is amazed that Goku wants to stand up to the challenge and that his mother was right about him. Future Trunks gives him the Heart Medicine and says he believes in Goku because his mother told him everything about him. Goku asks who Future Trunks' mother is and Future Trunks points her out, revealing her to be Bulma, which makes Goku fall over in disbelief that Bulma and Vegeta would end up together. Future Trunks explains that Bulma and Yamcha broke up and Bulma eventually falls in love with Vegeta, but they never married (this dialogue is changed to "he can never admit that he loves her" in the dub). Future Trunks does not remember his father, as he was an infant when he was murdered and is happy to meet him. Future Trunks asks Goku not to tell Bulma nor Vegeta about his identity or else they may not conceive him. Goku promises not to, and after thanking him for everything, Future Trunks leaves. Piccolo has heard the conversation with his enhanced ears, and tells the story to the others, without revealing anything that could endanger the birth of Trunks.
They discuss what do to in the three year before the scheduled fight with the androids. Goku reveals to the group that he narrowly escaped from Namek just seconds before the planet was destroyed. He tried to escape in Frieza's spaceship, but it would not start up. He then spotted a space pod belonging to the Ginyu Force and escaped in it. He ended up landing on the Planet Yardrat, and the creatures living there nursed him back to health and gave him their planet's outfit. Goku also says that he learned a new technique from them, the Instant Transmission. He shows it by teleporting to Kame House, stealing Master Roshi's sunglasses, and teleporting back in mere seconds. Piccolo suggests that they take training seriously this time. Bulma suggests that they use the Dragon Balls to locate Dr. Gero's Laboratory and destroy the androids, but Vegeta says that it would be too late by then. Goku decides that they should train their hardest and give all they got when the day comes. Vegeta tells Goku that he will fight the androids too, but once it is over, the two of them will settle the score and leaves. With that, everyone departs and begins training.
The remainder of the Trunks Saga shows the training that takes place by many of the Z Fighters in order to prepare for the arrival of the Androids. One significant event is the developing relationship between Vegeta and Bulma, even though he tends to shrug off her affection. There is also an ordeal Goku and Piccolo face on Chi-Chi's request: a driving test in order to receive their driver's licenses. However, their inability to acquire a driver's license is the least of their worries when the day of the android invasion comes. It is possible that a few Dragon Ball movies, including Dragon Ball Z: Cooler's Revenge (even though the movie was released between episodes 99 and 100), take place between episodes 124 and 125.