This is an alternate timeline that splits off right before the last three episodes of season four, enjoy.
(Most of this chapter is just setting the story up, a rough summary, the real story starts in the next chapter. the first and last paragraphs are the only new bit.)
Ulrich sat staring out the window, watching a single cloud move across the pale blue sky. The teacher was saying something about the Hundreds Years' War, but he didn't care, not really anyway. History always seemed so far away when Ulrich thought about his everyday life. Yumi loved history. When he complained about it this morning, she had said that knowing the past prepared you for the future.
Nothing could have prepared five of Kadic Academy's students for Xana; A malicious and highly advanced computer program whose sole purpose seemed to be total human destruction. These five students, the only ones who knew the truth, were Ulrich Stern, Yumi Ishiyama, Odd Della Robia, Jeremy Belpois, and Aelita Stones. Originally there had only been four students enrolled who knew, seeing as Aelita was found in Lyoko, a virtual world inside the supercomputer that Xana used to attack the school and surrounding area, and had only recently been materialized. A sixth Lyoko Warrior who had only been with the group for a short time was William Dunbar. William's curiosity and determination had lead him to the abandoned factory and the supercomputer inside. Yumi warned the group that William couldn't be trusted, but they desperately needed the help so they let him on the team. On his first trip, William went off on his own and was captured, and he had been stuck on Lyoko under Xana's control ever since.
Aelita's story was just as exciting but in a different dismal way. Aelita had come to live in the city with her father. Her father, Franz Hopper was a note worthy computer scientist, who built Xana and Lyoko to destroy a military project known as Carthage that was designed to disrupt enemy communications. He built Lyoko as a sanctuary for himself and Aelita but Xana trapped them both there. He was lost to the digital sea, at least that was the assumption, until recently when he came to Lyoko to save Aelita. Most of the rest of Aelita's past remains broken and fragmented.
In recent events Xana had been freed from the single supercomputer and now had access to supercomputers around the world, and at each a new duplicate sector of Lyoko was created and could be accessed through the digital sea. They were called Replikas. They could be accessed by using a ship called the Skidbladnir or Skid for short. They needed to shut down all of the Replikas before they could completely destroy Xana and focus on retrieving Franz Hopper from the digital sea.
The school bell rang making Ulrich jump, scattering his thoughts to the wind. The rest of the class filed out quickly. Ulrich stood and slung his book bag over one shoulder, following everyone out.