|Lost At Sea|
|Air Date (US)||August 28th, 2007|
|Air Date (France)||August 27th, 2007|
|Written by|| Hervé Pérouze|
|Storyboard by|| Paul Beneteau|
|Preceeded by||The Lake|
|Followed by||Lab Rat|
At Kadic, Hiroki asks Ulrich how to get Milly to notice him. Ulrich suggest giving her a story since she's a journalist. Hiroki then steals Yumi 's dairy only to be pursued by an angry sister. He reaches campus and drops the diary as he is running. Odd and Ulrich pick it up and hide it. They tell Hiroki where it is after he shakes off Yumi so he could dig it up and return in to Yumi. Upon unearthing the diary, he finds all the pages gones.
Coincidentally, Yumi appears and sees Hiroki holding the pageless book. She gets mad and states that she'll never forgive him for it. During a battle in the Digital Sea, Yumi's Nav Skid is separated from the group and she becomes lost. Unable to find her with the Skid's sonar down, Jeremie forces Aelita to withdraw until they can locate her properly. Meanwhile, Yumi's Nav Skid is running low on energy just as she finds herself under attack and it's up to Jeremie and Aelita to find her in time, before she will be lost at sea forever.
- The original French name of this episode is Torpilles virtuelles.
- This is the first episode were the Lyoko Warriors have new outfits.
- Introduction of William's personal Nav Skid rip off, the Rorkal.