The episode begins with Jeremie having difficulty making progress on Aelita's antivirus. Yumi and Ulrich come into the Lab to check on him, and Yumi assures him that finding Aelita's antivirus, as well as decoding Franz Hopper's diary, is only a matter of time. When Jeremie begins to doubt his abilities, Ulrich suggests he takes a break and invites him to Short Film Night at school, where Odd will be screening an original film for the first time.
That evening, a classroom is converted into a makeshift theatre, and Mr. Chardin welcomes the crowd of students and faculty to the Kadic Academy Cinema Club's Short Film Night. Odd's film, titled "Natural Grandeur", is revealed to be a parody of Kaijū films, in which a gigantic Sissi wreaks havoc on Kadic's campus. The crowd appears to enjoy the movie, and laughs throughout. Sissi, however, is embarrassed by her portrayal in the film, and abruptly stops the projector. As Sissi berates Odd, a spectre emerges from the projector and possesses her, causing her to act strangely and speak incoherently. Sissi is taken to the Infirmary and the Film Night is cancelled.
Yumi wonders whether or not X.A.N.A. may be involved, and the group decides to go to Jeremie's room to perform a Superscan. However, when the Scan does not detect any activated towers, and the school nurse announces Sissi is in perfect health, the Lyoko Warriors deem the incident a false alarm and go to bed. But later that night, Sissi escapes from the Infirmary and enters Jeremie's dorm, where she manages to write down a coded message for him before Jim arrives to take her back to the Infirmary.
The next morning, Jeremie asks Ulrich, Odd, Yumi, and Aelita to come to the Factory, where he tells them about Sissi's message. He then decodes the message using the Supercomputer, revealing the phrase "I can help you". The group revisits the possibility of a X.A.N.A. attack, but because Sissi's behavior does not match X.A.N.A.'s current modus operandi, Jeremie deduces that they are being contacted by an unknown third party. To know who, or what, is trying to communicate with them, they must speak to Sissi again.
Ulrich is still not convinced of X.A.N.A.'s innocence, so he and Aelita are sent to the Forest Sector on a reconnaissance mission to confirm X.A.N.A. is not involved while Odd and Yumi go back to Kadic to retrieve Sissi. However, when they arrive at the Administrative Building, they discover Sissi has been sent to The Hospital, and Jeremie tells them to follow her and bring her back to the Factory. Meanwhile, on Lyoko, Ulrich and Aelita discover a glowing white tower. As they discuss the tower's meaning, two Megatanks appear behind them. Aelita uses her Creativity to avoid them, but it is then revealed that she and Ulrich were not the Megatanks' intended targets; the monsters travel to the white tower and begin firing at it. Curious about the white tower's origin, Jeremie asks Ulrich to stay and protect it.
At the hospital, Sissi is attended to by a doctor. As Yumi and Odd walk to her room, Jeremie calls them to say X.A.N.A. has just activated a tower in the Ice Sector. Suddenly, screaming is heard in Sissi's room, and it is revealed that X.A.N.A. has possessed Sissi's doctor. The doctor turns on Yumi, and the two women struggle violently until the doctor is incapacitated by Odd, who shocks her with a defibrillator. Yumi and Odd then smuggle Sissi out of the hospital in a wheelchair.
On Lyoko, Ulrich battles the Megatanks. When Jeremie tells him and Aelita about the activated tower, they must decide whether or not the abandon the white one. Jeremie tells them to hold their position, but Aelita leaves on the Overbike, confident she that can deactivate the tower on her own while Ulrich stays and defends the white one. Ulrich is prevented from stopping her when two more Megatanks arrive.
Yumi and Odd reach the Factory bridge with Sissi. The possessed doctor arrives in a stolen ambulance and attempts to run them over, but misses, driving through the large doorway and crashing onto the Factory's main floor. The doctor attacks again, but Yumi, Odd, and Sissi manage to make it into the elevator unharmed. When they reach the Lab, Sissi takes control of the interface and Yumi goes back up to the main floor to intercept the doctor. Meanwhile, on Lyoko, Ulrich uses his Triplicate ability to defeat all but one of the Megatanks.
In the Ice Sector, Aelita is knocked off the Overbike when three Krabs appear, and Jeremie hurriedly sends Odd to Lyoko to rescue her. Ulrich is devirtualized by the last Megatank, which then returns to attacking the white tower, shutting it down and causing Sissi to be knocked unconscious. Meanwhile, the Scyphozoa appears and advances on Aelita, who is pinned by the Krabs. Odd arrives in time to defeat the Scyphozoa and escort Aelita to the activated tower before being devirtualized. On Earth, Ulrich goes up to help Yumi, who is losing her fight against the doctor. Aelita deactivates the tower, ending the doctor's possession and saving Yumi.
Some time later, Jeremie enters his dorm to find Aelita, Yumi, Ulrich, Odd, and Sissi already inside. Sissi angrily rejects an offer to star in Odd's next short film before walking out of the room. Now in private, Jeremie reveals to his friends that he has deciphered all the data Sissi gave him while she was possessed, and shows them by playing an audio file on his desktop. The message says "I can help you… I want to enter into contact with you now… I am Franz Hopper". Everyone, particularly Aelita, is shocked.
- The French name of this episode is the same as the English.
- While Sissi is possessed, she is actually speaking backwards. When the episode is played backward, it is revealed that she repeatedly says "I'm Franz Hopper. I can help you. I want to enter into contact with you now."
- When Sissi, Odd, and Yumi were being chased by the possessed doctor on the factory bridge, Odd makes a reference to Doctor Doom, a villian from Marvel Comics, saying "Let's get out of here! Here comes Doctor Doom!"
- Since a Return to the Past was not launched in this episode, it remains unknown what the Lyoko Warriors did with Sissi's doctor and the damaged ambulance after Aelita deactivated the tower.
- However, it may be likely that a Return to the Past really was launched in this episode, just off camera.
- The hospital room Sissi is admitted to, room 237, is the same room Odd and Ulrich are assigned ten episodes later in Temporary Insanity.