I'll wager that it's something along the lines of the "Garage Kids" pilot, but more refined.
My theory is that the gang is messing around in the park or in the basement, when they uncover the secret passageways. Being naturally curious, they investigate, and find the factory. Certainly Odd and Ulrich would mess around in there, while Jeremie would find the elevator and wonder why something that high-tech is doing in a run-down dump of a factory. He eventually cracks the code on the pad, and discovers the lab. (Solo)
After making the inital discovery of the lab and the supercomputer, he and the others use Lyoko as a virtual game area, messing around and dueling each-other, until Aelita is discovered watching them. They follow her into a tower, where the kids gain her trust and coax her out.
Finally able to see her, Jeremie is smitten and immeadiately tries to help Aelita. He hooks up his system at Kadic to the Lab's network, unaware that he is allowing X.A.N.A. access to the outside world for the first time in years. Stretching his abilities, X.A.N.A. launches an attack on Earth, and sends out the first group of monsters to destroy the meddling brats.
Aelita, realizing the threat, persuades Jeremie that the misfortune happening on Earth is X.A.N.A.'s doing, and convinces the rest of the gang to rid Lyoko of the monsters. After a rather clumsy initital assault, Jeremie rallys the gang and gets them to work together, allowing Aelita to reach the activated tower and foil X.A.N.A's world domination bid. (Whatever it may be.)
And the rest is history, as the saying goes!