Thirty years have passed since they beat xana, and everyone has settled down. Xana escapes and kills the group, leaving their kids to go back into time and help their parents defeat xana, without revealing who they are.
There's a lot that happens. First, someone gets blasted back in time. Then that someone learns more about Lyoko and becomes dedicated to stopping XANA. And then there's the supercomputer nearly blowing up and then Ulrich and Yumi got...Okay, let's just start here. One day, Sissi snuck into the factory. Reviewing helps me write better.