Dr. Gero was a brilliant scientist but also a lunatic bent on taking over the world. As the leader of the Red Ribbon Army, his plans were foiled by the young Goku.
Gero never lost sight of his grudge and began developing androids and spying on Goku and other fighter types in plans for his revenge. In order to extend his life, he turned himself into an android. He attempted to exact his revenge on Goku with Android #19, but underestimated his opponents because he did not calculate the ability developed off world known as Super Saiyan. After Vegeta killed Android #19, he decided he was going to attack Android #20 next. Gero attempted to escape so that he could awake Android #17 and #18 After running into Piccolo, and fighting him, Gero makes it to his lab.
In a last ditch effort, Gero awakened his dangerous, uncontrollable creations Androids #17 and #18. When he awoke them, they acted so polite and kind that Gero was surprised. However when he wasn't looking, #17 stole the control device used to shut them down and destroyed it. Soon after, #17 punched him through the stomach, hit his head off and stomped it.