The episode begins with Sissi reading the school newspaper and finds out that, when the academy was built in 1905, a construction worker named Leon Corbet was buried alive in the Boiler Room. During next class she gives a note to Ulrich to meet her in the boiler room at midnight.
That evening Jim gives a military like speech (which he totally confuses) about the different floors for boys and girls. At half past eleven Ulrich sneaks out of his room and goes down to the boiler room. There he is greeted by Sissi who ask him for a password. Ulrich answers “Ehh... You didn't give me one”, and Sissi lets him in. Inside the room he finds Herb and Nicholas sitting in a pentagram, made by red paint and candles. Sissi ask Ulrich to step in to the "pentathlon" with her so the can put the spirit of Leon Corbet to rest by communicating with him. Sissi drags Ulrich with her into the pentagram and starts the ceremony. Herb and Nicolas starts rattle and Sissi tries to summon the ghost. Then two electrical sparks comes from a wire and enters the boiler. Sissi believes that she actually manages to summon Corbet. Then a spectre appears and it scares Herb and Nicolas off. Sissi still believes that it is the ghost of Corbet and tries to speak to him, but Ulrich realises the danger and grab Sissi’s hand runs away with her.
Meanwhile Herb and Nicholas tell Jim about the ghost, but he does not believe them and gives them four hours of detention. Ulrich runs of to this room and asks Sissi to go to her, but she is too scared to be alone. Ulrich agrees to let her in to the room and then he leaves her with a sleeping Odd. Odd wakes up and when he sees Sissi he asks if it is a nightmare. Sissi asks Odd if she can sit near him. Ulrich, thinking that the ghost was XANA’s doing, wakes up Jeremie. Jeremie checks the Superscan, but no tower is activated. Ulrich then says “It looks like we have a real ghost on our hands then.”
Suddenly Jim, who is patrolling the Dormitory Building, hears strange noises. Thinking it is a student he shouts that someone is going to be in real trouble. Then something invisible grabs his leg and pulls him towards the ceiling. Sissi is telling Odd about the ghost, but is interrupted by Jim’s scream. Then run out of the room to find Ulrich and Jeremie watching their gym teacher floating in the air. Sissi starts to scream and Jim is released and falls to the ground. The three from team Lyoko run to Jim’s aid. Then they here the noise again and this time Odd’s leg is grabbed. Ulrich catches his hand and they both are dragged down the hallway. Jeremie finds a fire extinguisher and sprays the invisible ghost. It becomes visible and then escapes. Jeremie confirms that the ghost is XANA’s and they need to pick up Aelita and head to the Factory.
Sissi, who has hidden her self in a closet, is visited again by the ghost, but it ignores her in the search for team Lyoko. Ulrich calls Yumi who is reading in her bed and tells her to head to the Factory. Jeremie enters Aelita’s room and is just going to tell her about the attack, when he is strangled by the ghost. Ulrich finds a ruler and stabs the ghost so it disappears. The team heads to the sewer access in the park. When they come to the factory Yumi is already there waiting for them. Jeremie finds that the superscan is bugged by XANA so it can not find the tower. Aelita and Yumi heads to Sector 5 to fix the scan, Odd is searching the other sectors for the tower and Ulrich stays to protect Jeremie. Jeremie sends the Team to the Desert and Odd start searching from the Overboard while Aelita and Yumi heads for the Transport Orb. Right after they have been transported to Sector 5, the spectre appears in the Lab and Jeremie and Ulrich escapes. Odd finds the desert normal and heads to another sector. Meanwhile Yumi and Aelita are stuck on a pillar and several Creepers are heading towards them. Aelita uses her power to create a platform and Yumi uses her telekinesis to make it move, but the countdown is running low. Yumi presses the Key, but it stops halfway and now the need to leave the Carthage quick before it closes down. In the Factory, Jeremie and Ulrich is hiding from the ghost. In the Ice Barrier Odd spots a Tarantula and follows it. Aelita manage to get to the Arena before the walls close, but Yumi is smashed between them and devirtualized.
Ulrich attacks the ghost with an electrical cable and manage to force it away again. They hurry back to the Lab. Odd follows the monster through a ravine and find the activated tower. The tarantula spots him and starts to fire. It hits Odd’s tail so it starts to burn and another tarantula appears. Right in front of the elevator Jeremie is attacked by the ghost again and Ulrich attacks it with Pencak Silat, but it has no effect. Odd gets hit by a laser shot and is devirtualized. In the Scanner Room he finds Yumi and she tells him that Aelita is alone on Lyoko. XANA attacks her with the Scyphozoa and she calls for help. Odd hears it and tries to get the Supercomputer to work. With Aelita’s help he manage to enter SCIPIO and the transporter rescues Aelita a moment before the Scyphozoa was stealing her memories. Odd tells Aelita about the tower and then he starts to search for Ulrich and Jeremie.
Ulrich stills try to attack the ghost, with no success and is also captured. The Orb drops Aelita right in front of the tower and the two tarantulas. Aelita manage to run past the monsters and into the tower. Odd tries to attack the ghost, but instead he is knocked unconscious. Aelita enters the Code and the ghost disappears.
Back in the Lab, Jeremie decides to make a Time Reversion to fix the broken superscan.
Back in the past. Sissi tries, again, to summon Corbet’s soul. This time Odd is hiding behind the boiler and pretends to be the voice of the ghost and scares Sissi and her gang. Ulrich and Odd leave the room laughing, but then we hear a sort of ominous howling and the boiler hatch closes on its own.
This episode contains examples of:
- Combat Tentacles: The spectre XANA deploys in this episode makes use of these.
- The Juggernaut: The specter XANA summons to keep the others away from Lyoko is pretty hard to get rid of.
- Know-Nothing Know-It-All: Sissi when it comes to speaking to the dead, she thinks "Ying" and "Yang" are "Ping" and "Pong", which makes Ulrich quip that "she's missing some paddles".
- Missing Mission Control: Jeremie has to go run and hide with Ulrich to evade the Specter, which causes some issues as Yumi doesn't hit the switch in time.
- Needle in a Stack of Needles: Odd has to find the activated tower in the sectors, as Jeremie's super scanner program doesn't work.
- Oh, Crap!: Aelita gets cornered by a Scyphozoa after Yumi dies by being crushed by the hallways. Odd, of all people, has to type in the password to get the transporter to Aelita.
- Pajama-Clad Hero: Probably qualifies, as Jeremie tries to fight of the XANA Specter. Only Yumi and Ulrich are wearing clothes, and everyone is in pajama's or a nightie.
- Paranormal Episode: It's actually a XANA specter, but Sissi doesn't know this. The end of the episode reveals that the ghost is real, but no one is aware.
- Real After All: While the "ghost" that appears earlier in the episode is just a specter created by XANA, Leon Corbet is implied to actually haunt the boiler room at the end of the episode. A Ghostly Wail is heard, followed by the boiler room door closing on its own.
- This particular event happens when XANA's spectre first appears, causing everyone, including the Lyoko Warriors (temporarily), to think the spectre was Leon.
- Trust Password: Sissi requires one for Ulrich to enter the boiler room. But since there was no password that Sissi gave him, she lets him in anyway.
