his three friends Odd Della Robbia, Ulrich Stern, and Yumi Ishiyama, who are virtualized into Lyoko to save both worlds from the sinister virtual entity. They achieve this by escorting Aelita to various Towers on Lyoko, which serve as interface terminals between Lyoko and Earth. Once the Tower is deactivated, Jeremy can launch a "Return to the Past" program, which sends the world back in time to undo any damage caused by X.A.N.A., while anyone scanned into the Supercomputer retains their memory of the events. In "Code: Earth," Aelita is finally materialized, but the group discovers that X.A.N.A. had planted a virus inside of her that will kill her if the Supercomputer is turned off. They realize that they cannot destroy X.A.N.A, or Aelita will be destroyed along with it. In Season 2, Aelita adjusts to life in the real world, while Jeremy attempts to develop an antivirus program to liberate her from X.A.N.A.'s power. On Lyoko, a fifth sector is discovered and the group explores more of Lyoko's secrets and mysteries. The gang begins to uncover information about a mysterious man named Franz Hopper, who went missing ten years ago. He supposedly created the Supercomputer, Lyoko, and X.A.N.A., and is eventually discovered to be Aelita's father. They finally find out that Franz Hopper is indeed alive somewhere, hiding in the uncharted parts of Lyoko to avoid X.A.N.A. further. All the while, X.A.N.A. attempts to steal Aelita's memory to gain the Keys to Lyoko and escape into the internet. At the end of the season, the group discovers that Aelita is actually human and does not have a virus, and instead is missing a fragment of herself. In "The Key," X.A.N.A. tricks them with a fake and succeeds in stealing Aelita's memory and escaping the Supercomputer. Aelita appears to perish as a result but is revived when Franz Hopper restores her completely, along with her missing fragment: the memories of her life on Earth before she was virtualized on Lyoko. Season 3 shows that since succeeding in escaping the confinements of the supercomputer, X.A.N.A. targets the virtual world itself by destroying each of Lyoko's surface sectors, until only Sector Five is left. Initially reluctant, the Lyoko Warriors decide to invite William Dunbar as the sixth member. However, shortly after being virtualized, he is possessed by X.A.N.A.. Shortly after, he destroys the Core of Lyoko, destroying the entire virtual world and