Character Description
 Plot Involvement
Android 20 first appears along with Android 19, attacking a city in search of Goku and the other Z Fighters. Accompanied by Dr Gero, the first fighter to find them is Yamcha, who Dr Gero almost kills before Piccolo, Tien and Goku catch up. Goku convinces Dr Gero to fight away from the city so they won't endanger anymore lives and the group of them fly off into the wilderness. Goku says he will fight first as they are after him so Dr Gero has Android 19 fight him, whilst the others watch.
The fight seems reasonably well matched, even with Goku as a Super Saiyan. However, #19 begins to get the upperhand when Goku begins to suffer from his disease of the heart, causing him to weaken. #19 has beaten to the point where he is about to kill him when they are unexpectidly interrupted by Vegeta, who shows up out of no where. Vegeta takes over the fight, showing off his new ability to turn into a Super Saiyan. The Android tries to absorb Vegeta's energy but is unable to as Vegeta tears off both his hands, effectively disabling him. Despite trying to make a run for it, Vegeta doesn't spare the Android and destroys him completely with a Big Bang Attack. Realising that Vegeta, and the others, are much stronger than he had predicted, Dr Gero makes his escape, having an advantage in that as an android, he does not project ki.
Dr Gero manages to make it back to his lab where he activates Androids 17 and 18, whilst Vegeta and the others have caught up with him and are outside the entrance to the lab. Vegeta blasts down the door, but as Dr Gero orders #17 and #18 to attack, #17 turns on him and kills him.
Super 17 Saga
Dr Gero appears again much later on in Hell where he runs into Dr Myuu, a fellow scientist who harbours the same kind of hatred for Goku. The two of them collaborate and build another Android 17 in Hell with the intention of having this one combine with the one that still walks on Earth.
The Android 17 they created communicates with the one of Earth and is able to open a portal between Earth and Hell, allowing the three of them, amongst several other residents of Hell, to escape to Earth. Dr Gero and Dr Myuu wait until the Android 17 they created to combine into one Android - Super 17. But just as Vegeta and the others show up to fight the new Android, Super 17 turns on him, having been under Dr Myuu's control this whole time, and once again Dr Gero dies at the hands of Android 17.
Originally, Android 20 was known as the head scientist of the Red Ribbon Army, Dr. Gero. After the Saiyan Goku wiped out the Red Ribbon Army, Gero was able to escape his demise, and withdrew into the shadows, to form a master plan to eliminate Goku. He created many androids and cyborgs during this time, of which the vast majority turned out failures. The first we saw was Android 8, who appeared in Dragonball. But instead of killing Goku, he ended up befriending him. Gero kept trying though, and in Drgonball Z, he again unleashes several androids and cyborgs after Goku. The first is Android 19, who was not only created to kill Goku, but to also turn Gero into a cyborg himself, for the purpose of gaining eternal life. After he and #19 proved no match for Vegeta and Piccolo, #20 retreated and activated Android 17 and Android 18, who in reality were not androids, but cyborgs instead, given they were originally human. These cyborgs however, possessed the strength to beat the Z-fighters, but refused to obey #20, and ultimately killed him.
Secretly however, Gero had also been working on his master project, a creature named Cell. A being that was composed from the cells of the mightiest warriors the universe had ever seen, including Goku, Vegeta, Piccolo, and even Frieza. Cell's main goal was to absorb #17 and #18 into his being, for the purpose of achieving his perfect form, a form which, as Cell was told by Gero, had no boundaries. After Gero was killed by #17, his master computer finished the project for him. After his death, Gero appears one more time in Dragonball GT, where he teams up with another brilliant scientist, Dr. Myuu to create a new #17, to fuse with the original one on Earth. When this merger was completed, a super warrior was created, named Super Android 17, who was ultimately killed by Goku.
In the non canonical 7th Dragonball Z movie, three more androids created by Gero's master computer appear, namely: Android 13, Android 14 and Android 15. They ultimately merge together into Super Android 13, but again it is Goku who thwarts Gero's efforts.