CL reverse AU

Au for when Jeremie Belpois opened the Supercomputer he met XANA instead, who traps him in a scanner and transports him to Lyoko to possess him.  When well-meaning Ulrich Stern helps Jeremie out at the vending machine, XANA-fied Jeremie lures Ulrich to Lyoko with the story of a small defenseless girl trapped in there...and possesses him too. Yumi Ishiyama and Odd Della Robia are possessed in quick succession. 
However, XANA's power is limited and he can only possess these 4 kids with red towers. That's where Aelita comes in, and she hurries to deactivate these towers when XANA sets these kids to wreak havoc and sabotage on all sorts of computer systems and electrical systems, while dodging monsters XANA sends to deal with her on Lyoko. By deactivating the towers, the kids are temporarily freed, but suffer loss of memory and severe headaches. Whenever an attack is launched, XANA sets Jeremie Belpois on getting Aelita out of Lyoko to stop her from interfering, and to kill her for good; so it's a fight for the world and for her existence. XANA also uses his new warriors to learn more about the human race. Probably to conquer them better. 

Later on, William Dunbar becomes interested in Yumi and starts getting too interested in the XANA warriors' activities. Instead of disposing of him outright, XANA plans to possess him too. Aelita guesses XANA's plan, and once William is transferred to Lyoko, Aelita launches a surprise attack to distract the XANA warriors and Scypozoa, and get William to a safe tower. 

She tells him everything. She has to, because it's not like William can actually return to earth when the XANA warriors could be triggered any time to get the jump on him. They team up in an attempt to defeat the XANA warriors, who are now exploring the digital network to colonize every supercomputer XANA has his eye on, and to figure out how to get back to their normal lives.