Code Lyoko Seasons 1-4

Jeremie Belpois, a profoundly gifted and intelligent thirteen-year old boy attending boarding school at the fictional Kadic Academy, one day discovers a quantum supercomputer in an abandoned factory near his school. Upon activating it, he discovers a virtual world called Lyoko and Aelita, a young girl trapped inside it.

Afterward, unusual events begin to occur at school. Jeremie learns of X.A.N.A., an autonomous and sentient malevolent and rogue artificial intelligence/multi-agent system who also dwells within the supercomputer whose goal is to take over the real world. Jeremie soon forms a goal to materialize Aelita into the real world and stop X.A.N.A. in his tracks. With the help from Jeremie's best friends and classmates-Ulrich Stern, Odd Della Robbia, Yumi Ishiyama, and Aelita-the group goes to Lyoko in hope to save both worlds from the sinisterly evil virtual entity. With the exception of William Dunbar who joined the gang in the third season finale, but was possessed by X.A.N.A. once virtualized on Lyoko. He remained possessed until the episode, Down to Earth. He officially rejoins the group as the sixth Lyoko warrior in Code Lyoko: Evolution.