Code Lyoko - Season 4 Episode 12

Jeremie, Aelita, Ulrich, and Odd go with the rest of their science class on a field trip to an island in the middle of a nearby lake to study water-based plantlife. Yumi, not part of the class, remains behind to sit through two important exams. Jeremie's fears about XANA attacking at such an inopportune moment are realized when XANA creates an electricity-blasting sludge that attacks anyone who approaches the shore. Yumi is forced to go to Lyoko alone, and during her fight with William, is temporarily able to reach William's true personality. Though the evil William eventually reasserts himself, Yumi is saved by Aelita, who escaped the island. Aelita is able to shut down the tower and Yumi initiates a return trip to save the class from electrocution.