Code Lyoko [TV]

An evil and sinister sentient and autonomous, living and feeling, thinking and perceiving artificial intelligence/multi-agent program called X.A.N.A., is obsessed with world domination and destruction. To do this, he first must escape a highly complex and powerful supercomputer that imprisoned him. X.A.N.A. is able to attack the real world by activating towers on a virtual world called Lyoko, which act as links to the real world. When these towers are activated, X.A.N.A. is able to seize control of other computers and electrical systems as well as occasionally possessing organic life-forms or generating copies of them. In order to end an attack, Aelita must get to the activated tower(s) out of Lyoko’s five sectors to deactivate them, thus neutralizing the attack on the real world.
Once the danger is averted, the Lyoko Warriors can use the supercomputer to return to the past, leaving no one except themselves to remember any of the events that transpired. To complicate the situation, they must do this while ensuring their classmates and teachers are not killed (going back in time cannot bring back the dead), and deal with many clashes of personality at the same time. Once back in time, they can use their knowledge of the future to alter events in their favor. This usually entails taking steps to keep one of X.A.N.A.‘s attacks from taking place, though they do use the knowledge to prevent other unfavorable events that may not be related to X.A.N.A. As a downside, returning to the past adds a qubit to the supercomputer, greatly strengthening and doubling its power tenfold and therefore X.A.N.A.’s with each use (in the first two seasons only, as X.A.N.A.‘s connection to the supercomputer is lost by the third season).
[edit]Prequel
In the two-part prequel, twelve-year old Jeremie Belpois discovers an old abandoned factory containing a complex and powerful supercomputer. He reboots it, and learns that it contains a virtual world called Lyoko and a virtual humanoid being named Aelita, who does not remember her name and is thus called “Maya”. Strange and destructively powerful attacks begin to occur in the real world as a result of reactivating the supercomputer. Odd Della Robbia, Ulrich Stern, and Yumi Ishiyama become involved in these attacks, and Jeremie has no choice but to share his secret with them. The three are sent into Lyoko using three scanners to stop the deadly virtual force attacking their world. Once the attacks are stopped, the group discovers that all of the destructive havoc caused is all X.A.N.A.’s doing, an evil and powerful sentient and autonomous, thinking and feeling artificial intelligence, which wants to dominate the real world. “Maya” learns that her actual name is Aelita when she is deactivating the tower, thus stopping and neutralizing X.A.N.A.’s first attack.
In “X.A.N.A. Awakens, Part 2” Jeremy discovers a return-to-the-past function in the supercomputer, but nobody knows who or what created the supercomputer, Lyoko, Aelita, or the evil and sentient artificial intelligence, X.A.N.A. Joined by Ulrich Stern, Odd Della Robbia and Yumi Ishiyama, Jeremy intends to materialize Aelita into the real world and the three become the Lyoko warriors, promising to the keep the virtual secret.