S04E11 — The Lake

Jeremy, 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 plant life. Yumi, not part of the class, remains behind to sit through two important exams. Jeremy's fears about X.A.N.A. attacking at such an inopportune moment are realized when X.A.N.A. creates an electricity-blasting sludge that attacks anyone who approaches the shore. Yumi is forced to go to Lyoko alone. She fights William and during their fight is able to reach his true mind and personality. Though X.A.N.A. soon manages to regain control of William, Yumi is saved by Aelita, who escaped the island on Sissi's bike. Aelita is able to deactivate the tower and Yumi claims that one day they will bring William home. She initiates a return trip to the past and the class is saved from electrocution.