Time Reversion — codelyoko

Return to the past, now!
–Jeremie

Temporal Reversion, known in-universe as "Return to the Past", is one of the greatest and most useful properties of the Supercomputer. It allows the user to return back in time to a point of their choosing. Just how far back it can go is unknown, but it seems to be limited to a maximum of roughly two days. Each use of this feature doubles the processing power of the supercomputer, which in turn increased X.A.N.A.'s potential power.

The only people who retain memories of the time reversions are those who have been digitally scanned or virtualized by the factory's scanners. Also, in the episode Virus, a different method of creating immunity is shown to exist, when Laura Gauthier modified the temporal reversion from the interface.

Use

The person at the Factory Interface usually activates a temporal reversion after an attack to erase any damage caused to Earth; this resets the world back to how it was before the attack and wipes the memories of everyone back to that point, except for people who have been scanned into the Supercomputer, who retain all their memories. Temporal reversions do not affect events or beings that are linked to Lyoko such as undoing possession or deactivating towers. Also, it cannot bring the dead back to life, as pointed out by Odd and Ulrich in Teddygozilla and False Start respectively.

The return to the past program was used more sparingly after the episode A Great Day, when it was revealed that each use of the program doubles the supercomputer's power, and thus makes X.A.N.A. stronger. Also, since Aelita was still not fully human at that time, she was still stuck on Lyoko when a return in time was launched. Additionally, the return in time does not deactivate the towers on Lyoko.

Jeremie is usually the one who activates the temporal reversion process, though all the other members of the team (except for William and Odd) are shown to be capable of activating it as well. Some of them have even attempted to use it for their own personal gain. For instance, in The Chips Are Down, Ulrich used it to win the lottery in a misguided attempt to prevent Yumi from having to move back to Japan, which got him kicked out of the group temporarily for increasing X.A.N.A.'s strength without permission. Franz Hopper, Aelita's father, used this feature 2546 times over the course of his research, becoming more and more mentally unstable with each return in time. However, it's more frequently shown for the group to use their second chance to be kinder and make better choices than they did before.

Temporal reversions can also be very dangerous; in Temptation, Jeremie used it with a headset that multiplied his intellect by 10. Every time he used it, it caused him serious damage to his mental and physical health. The design concept of this headset was created by Franz Hopper. Also, in the episodes The Pretender and The Secret, it is revealed that if Aelita (or anyone else on Lyoko) gets possessed by X.A.N.A. via the Scyphozoa, and a return to the past is launched without freeing the subject, it could have deadly consequences; Aelita (or any possessed person) would most likely end up under X.A.N.A.'s control for good.

In Code Lyoko Evolution, it is revealed that Laura modified the boundaries of the temporal reversion program during the episode Virus without Jeremie's knowledge to include herself to be unaffected by the memory loss effect. This seems to be the only alternative to remember events after a return to the past, which doesn't include the subject needing to be materialized into the Supercomputer. It was also revealed that the event can be specifically altered to erase the Lyoko Warriors' memories as well by focusing it solely on the targeted warrior if they would be too much of a risk for the group. In Mutiny, this was done by Jeremie to Laura, who completely lost every memory she had about the Lyoko Warriors, their fight against X.A.N.A., and Professor Tyron after she launched a preemptive attempt to destroy X.A.N.A. that nearly got William turned into X.A.N.A.'s slave once again.

Times When Used

Season One

Every episode (except for Frontier and Code: Earth)

Season Two

Season Three

Season Four

Season Five: Evolution

Video Games

Immune Entities

Some peoples' memories are immune to the return trip. This can be achieved by being run by the supercomputer, or being recognized by the supercomputer as someone not to wipe. The following individuals are immune:

Person	Status
Jeremie Belpois	Gained off-screen after X.A.N.A. Awakens.
Odd Della Robbia	Gained in X.A.N.A. Awakens.
Ulrich Stern	Gained in X.A.N.A. Awakens.
Yumi Ishiyama	Gained in X.A.N.A. Awakens.
Aelita Schaeffer	Gained sometime in 1994.
Waldo Schaeffer	Gained, presumably, on first use.
X.A.N.A.	Inherent to the system.
William Dunbar	Gained in Final Round, though it is stated in Double Trouble that pieces of the events of The Secret bled through for him in a dream.
William Clone	Assumed, since he was shown remembering behavior from an alternate timeline in A Lack of Goodwill.
Monsters	Inherent to the system.
Laura Gauthier	Gained in Virus; lost in Mutiny.

Trivia

Sophie Decroisette later clarified in an interview that the reason the group knew a return to the past can't bring back the dead was not because someone died offscreen between X.A.N.A. Awakens and Teddygozilla, but because Jeremie came to the conclusion from analyzing the program.^[1] Jérôme Mouscadet did however say in the same interview that had they been allowed to, they would have written an episode where a character died.
In the graphic bible used during production of Season 1, originally return trips to the past could only be activated by Aelita when she deactivated a tower.^[2] She was said to "fuse" with the tower and restore time to before X.A.N.A. activated it. This explains how returns to the past were activated in Laughing Fit and Routine, and why one wasn't in Frontier.
- Originally Aelita was also the one to give the group immunity to the return trips "so that they never forget that the Earth is in danger".
The return to the past was added to the series by Frédéric Lenoir, who was looking for a way to explain how X.A.N.A. could cause enormous damage to the real world, have other people witness it, and have the group be able to fix it without others becoming suspicious.^[3]^[4]
The return to the past function is never mentioned nor used in the Code Lyoko Chronicles series.
The effect likely turns back time all over the planet, otherwise the city and country would become out of phase with the rest of the world, and perhaps even does so throughout the entire universe, so as to preserve the Earth's relative position in it.
- This leaves the question of what happens to all the people who originally died during that day, since the program is able to recreate anything except life, and that would include bodies. Do they all appear to fall dead at the same time, at midnight in Paris, for example.