Code Lyoko - Season 2 Episode 8
Missing Link

During a mission to The Carthage Region, the Scyphozoa attacks Yumi, stealing her human DNA, having her trapped on Lyoko. To make matters worse, its picture day for her class and she will be suspended if absent. After some discussion, Ulrich gets the idea to have Sissi stand in for Yumi. Strangely, Sissi agrees and asks for nothing in return. During this, Jeremie tries to come up with a way to bring Yumi back. Aelita thinks of a way, but wont tell Jeremie about it. She only tells him once she gets to Lyoko, so he cant object. She begins to transfer her own genetic code to Yumi, which will let Xana take her memory at will, being exposed. Luckily, Odd and Ulrich get there in time to stop her. Meanwhile, Jermie figures out that Yumis DNA should be stored in XANAs memory, which is accessible through Sector Five.