Can you keep a secret?
Code Lyoko is an Animesque show that's written and produced in France in English and French for simultaneous release in its home nation as well as the US — and about 160 countries, including Japan (a rarity for a foreign animated show).
It follows the adventures of a group of students at a Boarding School who discover a supercomputer in the basement of a nearby abandoned factory. The computer has modules that allow people from the real world to "virtualize" into the Cyberspace of Lyoko, where an evil A.I. named XANA is apparently hunting down a good A.I. named Aelita. The computer also has the ability to effect a temporal recursion (in Layman's Terms, rewind time) up to 24 hours in the real world.
XANA manifests in Lyoko indirectly through a variety of monsters with various attacks and tactics, trying to capture Aelita and repulse her protectors from the real world. XANA also attacks the real world, which at first resembles the results of The Cracker, doing damage through various interfaces, but these attacks slowly gain a supernatural edge, including monsters in the real world, and possession of students and teachers.
The students often use the Return to the Past to repair damage and effect a kind of Masquerade to protect Aelita, however with the knowledge that death is permanent — not even time travel will bring them back. Later it is revealed that each Return to the Past makes XANA slightly stronger.
The five main characters (Aelita and her protectors) are occasionally referred to as "The Lyoko Warriors" or Team Lyoko. See the character sheet for individual descriptions.
There is a short (5 minutes long) film made in 2001 by the creators of the show, Garage Kids, that is considered a test pilot for the series. It introduces the main characters and some future elements of Code Lyoko, though there are differences both visually and plot-wise. Code Lyoko began broadcast in France in 2003 on France 3 and Canal J, and in the United States in 2004 on Cartoon Network. The TV series ended in 2007 after four seasons and 97 episodes (including a two-parter prequel).
As of summer 2011, a new twenty-six episode series was confirmed to be in production, titled Code Lyoko Evolution, to be released in fall 2012. Evolution will be a continuation of the original show, with live-action replacing the 2D segments. All tropes relating to this series should go on the Code Lyoko Evolution page.
This was one of my favorite shows as a kid and Im happy to see it's getting a sequal.