Code: LYOKO, on France 3 (France) / Cartoon Network (USA)
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In the real world (which is drawn in 2-D), there are some kids at a boarding school called Kadic, who can go to a virtual universe (which is in 3-D CGI), called Lyoko. At some point prior to the start of the series, these kids discovered an abandoned factory near the school, which has a supercomputer, and scanners that can virtualize them and send them to Lyoko. There's Ulrich Stern, Odd Della Robbia, and Yumi Ishiyama (the only one who lives with her parents rather than on campus; at this point I feel compelled not only to say that we kind of like her, but also to point out that we feel rather weird about kind of liking any of the girls on this show). There's also Jeremie Belpois, though he doesn't go to Lyoko. He's a computer expert and stays behind, monitoring activity on Lyoko on the supercomputer (or when away from the factory, on his laptop); and keeping in touch with the others both on Lyoko and in the real world (everyone has mobile phones, of course).
They have a friend on Lyoko named Aelita (who we kinda like), who Jeremie is trying to find a way to devirtualize and bring to the real world. There is also some kind of unseen virus called X.A.N.A., who activates these towers in Lyoko which somehow let him take control of various things in the real world to wreak havoc for some reason. To stop him, Aelita has to find the tower, enter it, and deactivate it, thus returning things to normal in the real world. But XANA. also sends different "monsters" to try to stop her. So the kids go to Lyoko (where their appearnces change to 3-D and they each have a different kind of weapon which they use to destroy the monsters) to protect Aelita on her way to the tower. The kids have a certain number of life points on Lyoko, and if the monsters' blasts hit them often enough, they get devirtualized and return home. Anyway, after everything is set right, everyone goes back in time (about a day, I think), using a "return to the past" program on the supercomputer, but these kids are the only ones who remember what's happened. In the new history, none of XANA's havoc has been wreaked. (It's also useful for preventing anyone from learning the truth about Lyoko and XANA and all that, because sometimes... people do discover the secret, and that would be a bad thing.)
There are other characters in the real world. Odd has a dog named Kiwi. There's a Phys Ed teacher, named Jim MoralÚs. He's always suspicious of them, and they often get in trouble with him. There's also a girl called Sissi Delmas (the principal's daughter, who we kinda like), who, though she's supposedly the prettiest, most popular girl in school, always hangs out with couple of loser guys, Nicolas and Herb, probably the only kids in school who truly like her (though she obviously doesn't think much of them). The three of them call themselves Sissi's gang. She has a major crush on Ulrich, who isn't interested in her, because she's a bratty, stuck-up jerk. Besides, Ulrich and Yumi like each other, though the two of them never seem to be able to tell each other how they really feel, though everyone around them knows. Jeremie and Aelita also like each other, and they seem a bit better at showing it than Yumi and Ulrich, though it still takes them awhile. Anyway, Sissi's gang and the Lyoko gang are always insulting and tricking one another (although Sissi wishes she could join Ulrich's group). There are also a couple of younger girls, Milly and Tamiya, who do video reports for the school's news program (though I think in season two they work at the school newspaper, I'm not sure; in season 3 they certainly seem to be back to video). Sissi also seems to enjoy picking on them just for being younger. I should probably also mention that in season 2 Ulrich has a rival for Yumi's affections in a new student named William Dunbar.
At the end of the first season, Jeremie finally finds a way to devirtualize Aelita, but XANA creates a virus that prevents her from staying in the real world full time. They want to just shut down Lyoko, thus destroying XANA, but the virus links them so Aelita would be destroyed, too. But Jeremie's always trying to develop an antivirus. Meanwhile, she will spend more time in the real world (where she's of course drawn in 2-D) in season two, and even enrolls at Kadic, assuming the name Aelita Stones, and claiming to be Odd's cousin from Canada.
Also at the start of season two, Jeremie has created vehicles for the Lyoko Warriors: the Overboard for Odd, the Overbike for Ulrich, and the Overwing for Yumi- although sometimes Aelita also uses one of the vehicles herself. Jeremie continues to try to make an antivirus, but is more successful with other developments. But XANA's making new monsters and getting stronger, too. The most important of the new monsters is the Scyphozoa, which always tries to steal Aelita's memories. Another important facet of the season is that Aelita starts having visions and nightmares, so there's a new mystery to solve. Which also involves a scientist named Franz Hopper, who apparently used to teach at Kadic. He also owned a now deserted house in the woods referred to as "the Hermitage," which is the site of some of Aelita's visions. Eventually they find some of Hopper's files, which are encrypted, and will take Jeremie a long time to decode. Further complicating things this season, they discover that returning to the past makes XANA stronger, so they'll have to try to do that less often now, but sometimes XANA still leaves them with no other choice. And I should say that there were in season one four different sectors on Lyoko: Ice, Desert, Forest, and Mountain. In season 2, a fifth sector, known as Carthage, or sometimes simply "Sector 5," was discovered, and would be very important.
At the end of season 2, XANA escapes from Lyoko onto the Internet. But all of Aelita's lost memories of her past are restored, and she's able to leave Lyoko full time. However, the Lyoko Warriors can't shut it down, because it's their only way to fight XANA. So, throughout season 3, XANA tries to destroy Lyoko, sector by sector, while also frequently attempting to destroy the core of Lyoko in Sector 5. Oh, I should also mention that at the start of the season, there were a couple episodes set in the past, that finally revealed how the kids first met each other and discovered Lyoko. Anyway, at the end of the season, the gang finally decides to recruit William to help them on Lyoko, but XANA ends up possessing him and using him to destroy Lyoko, and William himself along with it, apparently. Franz Hopper, meanwhile, who had been stuck somewhere in Lyoko, escapes onto the internet.
Season four begins with Jeremie and Aelita trying to recreate Lyoko (tres ironique), so they can track XANA and fight him, and also look for William, hoping to rescue him. Finally they manage to recreate Sector 5 (and before long the other sectors, though I don't recall them mentioning having recreated them, they just started going to them). William returns, still apparently possessed by XANA, and now capable of surviving the digital sea (which would kill anyone else if they fell into it, apparently). So now he tries to kidnap Aelita and throw her into it. He also commands XANA's monsters, and the gang often have to fight him on Lyoko. Meanwhile, Jeremie created a polymorphic clone of William, to convince everyone at Kadic that William is still around. In other news, Sissi decided to take over the Kadic newspaper and boss Milly and Tamiya around, but before long they quit. Also, the gang got slightly altered avatar bodies on Lyoko, and Aelita's even has wings. Plus, Jeremie and Aelita designed a submarine to explore the digital sea, which is apparently the internet. Sort of. Lyoko is like this sphere in the middle of the sea. They named their sub "Skidbladnir," after a Norse mythological ship, but most of the time they all just call it "the Skid." Anyway, while exploring they soon discover another sphere containing a reproduction of Lyoko, or at least one sector. Which Jeremie realizes must mean there's another XANA-infected supercomputer somewhere in the real world.... Eventually they'll discover multiple replicas of Lyoko, and every time they do, they'll have to transport from the replica to one of the other supercomputers and destroy it. And while there, their appearance will be... sort of a cross between how they'd normally look on Lyoko and on Earth. And that's about all I can say for the moment, as I didn't get a chance to see all of season four. I certainly hope to finish the series, someday.
Anyway, I just want to say that it's a pretty fun show, and interesting, and funny, and cute. Oh, and the theme song (either "A world without danger," or the original French version, "Un monde sans danger") is really catchy. I felt almost a little guilty about liking the show in the first season, but not at all in the second season and beyond, because the quality greatly improved. Oh yeah, I should probably also mention that considering the target age group for shows on Cartoon Network's Miguzi block, there's a surprising amount of adult situations, dialog, and lite fanservice. All quite enjoyable, if a little disturbing....