In the Factory, Jeremie and Aelita work on a virtual submarine, and repurpose a section of Carthage into a hangar for the vessel. Odd gives the submarine the code name Melanie, after which Ulrich teases him for naming the ship after a girl from school. On the way back to campus, Jeremie and Aelita inform the others that they must return to the Factory at four o'clock sharp the next day to complete the ship, or else its programs will become unstable and the entire project will be ruined.
Later that night, Aelita sneaks into Jeremie's dorm to do some last minute work on the submarine. However, she is caught by Jim, and she and Jeremie are given detention, making it impossible for them to return to the Factory on time. The next day, the Lyoko Warriors try in vein to prevent the detention from happening, and Jim takes Jeremie and Aelita to the campus library to serve their time.
While Odd and Ulrich try unsuccessfully to trick Jim into letting Jeremie and Aelita go, Jeremie's laptop alerts him to a large group of monsters, led by William, arriving in Carthage to destroy the submarine. However, when he and Aelita try to contact the others, Jim confiscates their phones and prohibits Jeremie from using his laptop.
Eventually, Jeremie communicates the message "SOS Factory William" to Yumi using Morse code, and she, Ulrich, and Odd travel to the Factory. Meanwhile, Aelita distracts Jim by claiming to need help with her math homework so that Jeremie can secretly use his laptop to message Yumi. At the Factory, Yumi, following Jeremie's instructions, sends Odd and Ulrich to Carthage to defend Melanie.
Jeremie has his laptop confiscated when Mr. Delmas arrives in the library and catches him communicating with Yumi. Meanwhile, Ulrich confronts William in the hangar while Odd battles the monsters. However, the boys are outnumbered, and the submarine continues to be attacked, its shields deleting, despite their best efforts. Desperate, Yumi calls Hiroki and asks him for help getting Jeremie and Aelita out of detention.
Hiroki and Johnny take Kiwi from Odd's dorm and let him loose in the library. Jim is distracted by the dog, allowing Jeremie and Aelita to escape. They arrive that the Factory just as Odd is devirtualized by a Manta, and Aelita and Yumi are sent to Lyoko.
Ulrich is devirtualized by William while Yumi and Aelita defeat the remaining monsters. As William attempts to destroy the last of Melanie's shields, Aelita attacks him, and he too is devirtualized. In the Lab, Jeremie succeeds in completing his work on the submarine just before the deadline of four o'clock, and the ship is officially completed.
Later, at the Hermitage, the five Lyoko Warriors discuss naming the ship. Aelita suggests the name Skidbladnir, remembering it from Viking Gods and Legends, a book her father used to read to her when she was young. The name is agreed upon, with Odd giving it the additional nickname "the Skid".
- The original French name is the same as the English.
- The Skíðblaðnir is a magical ship from Norse Mythology, said to be owned by the god Freyr. The ship is described as "the best of ships", and was able to reach its destination no matter what direction the wind was blowing.
- William is devirtualized for the first time ever in this episode.
- This is the only episode to have the Skidbladnir in the title card.
- The teacher that gives a lesson on Shakespeare near the beginning of the episode has not been seen since the season 1 episode Ghost Channel. She later makes her third and final appearance in Kadic Bombshell.
- Right before Aelita and Yumi arrive in the hangar, Ulrich is shown holding both swords. However, in an earlier scene, one of his swords had fallen down into an unreachable part of the hangar. When he is devirtualized in the next scene, he only has one sword again.
- When William is devirtualized by Aelita, he makes a small scream that makes him sound like he's not possessed by X.A.N.A. anymore.
The original episode summary can be found here.