Spectromania

When a kid goes up to hug Aelita, he leaves and comes back and does the same to Yumi. Jeremy immediately realizes that XANA has sent a specter to retrieve the codes it implanted into four of its greatest enemies to regain its full strength and power. So they go to Lyoko and attempt to deactivate the tower in the Desert sector while Odd stays at the school to hold it off, but the situation escalates when Aelita gets hit and is devirtualized. She guides Yumi from Earth, supposing that XANA's source codes can allow the others to deactivate towers. Her theory is correct and Yumi deactivates the tower, only to reveal another one masked by the first's signal; another spectre is somewhere on Earth.

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S01E04 - Madame Einstein