It was pretty cool, but I hate series that just drop you in the middle of an established narrative. The bad entity, Xana, fucks up something in the lyoko world which fucks something up in the real world that they have to fix. They end up fixing it, and then they go back in time before everything went tits up.
It's been many years since I've seen it, but there was never an apparent explanation as to what Xana was, how these kids built and/or happened upon this facility, how the lyoko world came to be, how it is they can go back in time, or numerous other things I'm probably forgetting.
Cool show though.
Shut your BF28/9 sound hole and listen up.
Fool me once, shame on you. Fool me twice? Strike three.