At Kadic, Ulrich, Aelita, Odd, and Jeremie meet up in the courtyard in the morning. Just before breakfast, Hiroki approaches them and asks to speak with Ulrich privately. Hiroki then confesses to having feelings for Milly Solovieff, and asks for advise on how to get her to notice him. Ulrich suggests talking to her about about her interests, namely journalism, and Hiroki interprets this suggestion to mean he should find her a story to put in the school paper. At breakfast, Ulrich, Odd, Aelita, and Jeremie discuss Hiroki's crush on Milly before Jeremie reveals he has developed a new program to search for William in the Digital Sea, and suggests they test it that afternoon.
Some time later, when Yumi sets her backpack down to use a vending machine in the courtyard, Hiroki steals her dairy, planning to find something revealing and newsworthy in it to show to Milly. Yumi catches her brother in the act and gives chase. While trying to escape his sister, Hiroki runs into Odd, who is walking with Ulrich nearby, before fleeing into the park. When Yumi finally catches him, it is then revealed that Hiroki accidentally dropped the diary when he ran into Odd, and that it is now in Ulrich's possession.
Despite Odd's persuasions, Ulrich resists the temptation to read the diary and, after classes let out, he attempts to give it back to Yumi. However, he is interrupted when Hiroki approaches her and apologizes to her for losing the diary. When Yumi does not accept Hiroki's apology out of anger, Ulrich and Odd decide to hide the diary in the park so that Hiroki will find it and reconcile with his sister. However, when Hiroki finally locates the book in a pile of leaves, the pages have fallen out, and only the cover remains. Yumi discovers Hiroki with her ruined diary and becomes even angrier, vowing never to speak to Hiroki again.
That afternoon, the Lyoko Warriors travel to the Factory, where Ulrich, Odd, Yumi, and Aelita are sent to Carthage. After they all board the Skid, Aelita pilots the ship out of Lyoko and into the Digital Sea while Jeremie implements the new program. However, a problem occurs, and Jeremie's program damages the Skid's sonar system. Without sonar, the mission is impossible, and Aelita turns the Skid around to return to Lyoko. However, before the Skid reaches its destination, it is attacked by three Kongres, and Ulrich, Odd, and Yumi are released in their Nav Skids to fight them off.
Once the battle is over, Ulrich and Odd return to the Skid, but Yumi does not, and it is revealed that Yumi was hit by a laser and knocked into a Hub, sending her to another part of the internet. With the Skid's sonar still damaged, the other Lyoko Warriors cannot search for Yumi, and have no choice but to return to Lyoko, and then back to Earth, so that Jeremie and Aelita can make repairs to the Skid. Meanwhile, somewhere in the Digital Sea, Yumi tries in vein to contact Jeremie and the others in her severely damaged Nav Skid. She looks back on her relationship with Hiroki, and regrets the anger she showed him earlier that day.
As Jeremie and Aelita work to locate Yumi on the Network, Ulrich and Odd go to the park, where they discover the pages of Yumi's diary were stolen by Kiwi, who had taken them and buried them in a separate area away from the cover. Ulrich takes the pages from the dog as he continues to worry about Yumi. In the Digital Sea, Yumi is approached by another Kongre, and manages to destroy it with her last torpedo. The explosion is detected by Jeremie and Aelita in the Lab, who recognize it as a sign Yumi is still alive. Aelita calls Ulrich and Odd back to the Factory as Jeremie triangulates Yumi's location. Despite the Skid's sonar still being damaged, Ulrich, Odd, and Aelita all volunteer to go on a rescue mission, and the three of them return to Digital Sea.
Meanwhile, Yumi is attacked by yet another Kongre. Now defenseless, Yumi is forced to flee, navigating through the internet until William arrives at her position in a specially made submarine of his own. However, when William fires at Yumi, she is defended by the Skidbladnir, which arrives at her side just in time. Yumi anchors herself to the Skid while Odd and Ulrich battle both the Kongre and William. Ulrich destroys William's submarine, and the newly reunited group returns to Lyoko together.
That night, after returning to Earth, Yumi enters her bedroom to find the pages of her diary lying on her bed. Hiroki then appears in her doorway, and apologizes again for stealing the diary, and Yumi, sobered by her experience on the Network, forgives him. It is then revealed that Ulrich secretly placed the pages in her room, climbing a tree into her house to do so.
- The original French name of this episode is Torpilles virtuelles, which translate to Virtual Torpedoes.
- William's personal submarine, the Rorkal, is introduced in this episode.