The episode opens with a rock climbing lesson in gym class. Jeremie does poorly climbing the rock wall and falls, after which he is chastised by Jim for being unathletic. Later, in the cafeteria, the Lyoko Warriors discuss the Skid. It is then revealed by Mr. Delmas that it is Fire Safety, Risk, and Prevention Day at Kadic Academy. Due to FSRP Day, all afternoon classes are cancelled and all buildings on campus must be temporarily vacated and locked as part of an evacuation exercise. After being persuaded by Aelita and the others, Jeremie agrees to take advantage of the lockout to go to the Factory to perform a test run of the Skid.
Just before the lockout begins, the group heads over to the Factory, and Aelita, Ulrich, Odd, and Yumi are sent to Sector Five. They are each energized into the Skid, and Aelita pilots the ship out of Carthage. Then, after they persuade Jeremie further, he allows them to dive into the Digital Sea for the first time. The dive is successful, and Aelita and the others explore the internet. Suddenly, Jeremie discovers a bug in the navigation systems on both the Skid and the lab's interface. Without a navigation system, the Skid loses its ability to return to Lyoko, and is effectively lost in the Digital Sea.
Jeremie reveals that the Skid can only stay in the internet for a short time, and that once it runs out of power, it will disintegrate, killing the crew along with it. Jeremie then reveals that he owns a CD containing subprograms that can debug the system, but that it is located in his dorm room. Aelita tells Jeremie to retrieve the disc, and he leaves the Factory. However, on campus, Jim denies Jeremie entry into the dormitory building due to the evacuation exercise, and he is forced to sneak in through a side entrance.
In the Digital Sea, the Skid comes across a large object, and begins to be pulled in by its gravity. Odd tentatively suggests they have somehow found Lyoko again, but Aelita does not recognize the object, and is forced to use a large amount of the Skid's power reserves to break away from it. On campus, Jeremie finds more teachers guarding the halls of the dormitory building, and climbs onto the roof to avoid being seen. After experiencing mild vertigo and nearly falling, Jeremie regains his composure the is able to scale the building's gables.
Then, two large eels approach the Skid, and Aelita attempts to elude them after they open fire. Meanwhile, Jeremie finds an open skylight leading to the attic, and reenters the dorm building, where he proceeds to his room and retrieves the CD. After being spotted by a firefighter, Jeremie escapes from campus and enters the manhole in the park.
Jeremie arrives back in the lab and begins the debugging process as Aelita releases the Nav Skids, allowing Ulrich, Odd, and Yumi to battle the eels in order to protect the Skid, which has lost a dangerous amount of power. When the debugging is completed, Jeremie sends Aelita the coordinates back to Lyoko. The Nav Skids reattach to the Skidbladnir, and Aelita escapes the eels, returning the ship safely to Lyoko.
Later, the Lyoko Warriors convene in the lab where they discuss the Digital Sea. Aelita and Yumi worry about Franz Hopper and William, respectively, and Ulrich wonders what the object they encountered was. Jeremie suggests the object was a Replika of Lyoko created by X.A.N.A. for currently unknown purposes. Then, when Jeremie explains he was forced to scale the roof of the dormitories to retrieve the CD, the others laugh at the irony of his poor performance in gym.
- The original French name for this episode is Premier voyage.
- The title card for this episode is the first to feature the Holoweb.
- Starting with this episode, most characters, including all of the main characters, receive new designs.
- The Skidbladnir, as well as the Nav Skids, are driven for the first time in this episode.
- This episode marks the first time the internet beneath the Digital Sea is seen.
- This is the first episode to feature a Replika as well.
- This is the first out of four episodes of the fourth season where William does not make an appearance.
- This episode is noteworthy for being one of the very few in which X.A.N.A. plays no part in the plot.