Jeremie has created a EMP bomb, which can release a person from X.A.N.A.'s control. "Bomb explodes, guy under X.A.N.A.'s control obliterated." Jeremie (in response to Odd's reply to Jeremie's long description to make it short) At Kadic Academy, Sissi comes up to Ulrich, Aelita, and Jeremie, insulting Yumi, William, and Ulrich all at once to make him jealous, or so it seems. Jeremie picks up an activated tower, and says to go to the factory, skipping Music Class. However, Jim stops them before they can go.
The Principal is seen going in the direction of Yumi, Odd, and William. Odd bides farewell saying that he has to get to Music Class with the others, while William states that he's lucky, as they have chemistry. The Principal grabs Yumi and Odd, after she greets him. William objects this behavior, and tries to stop him, but he throws William into the science building. Outside the school gates, a car's doors open automatically. The principal gets to the car, throwing Odd and Yumi there and closes the doors. Yumi pleads as Mr. Delmas is revealed to be possessed by X.A.N.A. William follows, limping but watches in despair as the car drives off. Later, Nicholas is playing a car racing video-game as William warns him that Mr. Delmas has "gone crazy", and tells other people the details about it.
In Music Class, the teacher is playing an instrument, and no one seems to care much. Nicholas then enters the room, telling the teacher about Yumi and Odd's predicament. "X.A.N.A.!", Jeremie says. The teacher does not believe it, but Nicholas explains that William said that. The teacher then goes to see. X.A.N.A. text-messages Jeremie. The message reads "Aelita at 2 'o clock in the Ice Sector to give herself up to the Scyphozoa. If not, Odd and Yumi will be liquidated." They stand up and try to go, but the music teacher opens the door and orders that everyone sit down immediately.
At a warehouse, Yumi and Odd wake up, and he comments that given the temperature, they must be in Siberia. Odd then tries to walk up to the principal, but he gives him a shock, sending Odd back to the wall. Back at Kadic, Jim announces that classes are totally cancelled for the day as the rest of the group try to sneak out. They notice them and they run for as if it were their lives, up a fence, around the track as Jim gets tired, and then the other people grab Aelita and Jeremie, as Ulrich makes it into the forest where the sewer cap is located. Jim crashes into a tree as Ulrich gets into the sewer, narrowly avoiding them. He gets a skateboard from the wall and grinds a rail as Odd explains to the principal that cold can kill them too. He them throws them a sweater to avoid from being encased in ice within the ultimatum before the right time. Meanwhile, Aelita and Jeremie are in the office beside the principal's, where they discuss with Sissi the entire details of what's happening to Mr. Delmas, While Yumi sees the cell phone.
By then, Ulrich is almost at the factory, as Yumi grabs the cell phone and tries calling Ulrich, while Ulrich gets to the elevator as Yumi's phone gets to his, as Yumi gives him some details, but The principal grabs Odd and Yumi's phone by attracting them with a magnetic charge and breaking them as Ulrich loses the signal, and he then sees the EMP bomb, as Jeremie and Aelita have told Sissi about Lyoko. Aelita says "It's not us you're helping, It's your father. You have to trust us. I'm begging you!" by that time, Sissi is called to the office. Ulrich then sulks a bit because Jeremie hadn't made an instruction manual for the EMP bomb, and pulls switches/ presses buttons until the bomb is turned on. At that time, Sissi says makes a remark that he father had been "locking himself in the basement" reguarly and she heard him say something about detonators, getting the detective to evacuate the school. Aelita, Jeremie, and Sissi then run to the factory as Ulrich calls them and Jeremie says there is only 15 minutes left until the ultimatum (thus the title of the episode).
At the factory in the control room, Sissi makes a comment about that she feels like she is in a James Bond movie. They locate the Scyphozoa. Ulrich tries doors at the refrigerator warehouse as the Ultimatum is up. Ulrich then finds them and uses the EMP bomb after Jeremie tells him that the bomb can also simply deactivate anything electronic, breaking X.A.N.A.'s control of Mr. Delmas for a bit of time. At the factory, Aelita has Sissi punch Jeremie in order to virtualize her to Lyoko. Once there, she is then chased as the others are running to the factory. Aelita then slides down in order to escape from X.A.N.A.'s monsters. One is completely devirtualized from damage as Aelita lands in front of a rock and walks back into the Scyphozoa. Back in the factory, Sissi wakes Jeremie up, asking about the Scyphozoa, describing it as a "giant squid" as Yumi and Odd are virtualized. They manage to cut the Scyphozoa's tentacles in time. Odd then shoots laser arrows at it, While Ulrich is taking a beating courtesy of the principal. as they go up an ice mountain, as Tarantulas materialize. they are devirtualized as the principal attacks Sissi and Ulrich. Aelita gets to Yumi in time as she falls into a cave with a tower. She then enters into the tower and deactivates it, knocking Mr. Delmas out cold. Sissi and Ulrich have a romantic moment where Sissi asks what will happen to her father with the kidnapping and such, and Ulrich assures Sissi no one - including herself - will remember anything, but he will. She kisses him, much to his and Odd's shock, who is calling Jeremie and says, "You can go ahead Einstein, I wonder which would be worse; being blind or having to see this." Jeremie then enters the code for a Time Reversion.
Time is reversed to before Sissi insulted them, in which after she leaves, Jeremie says "You're still thinking about what happened on the bridge?" Yumi then says "What happened on the BRIDGE?" Aelita also asks the same question. Ulrich says "Nothing". Yumi objects a bit, but Odd says "But nothing really DID happen on the bridge." as Ulrich meets up with Sissi as the bell rings. Yumi doesn't believe them, but Aelita figures that it's "just another boy thing" and Yumi somewhat jealously watches Ulrich with Sissi.
- The original French name for this episode is the same as the English.
- Chronologically, this episode takes place after Temptation and A Bad Turn, however it was aired weeks before either.
- In the scene with Yumi and Odd in the warehouse, the phrase "Killroy was here" can be seen. It's a phrase associated with the American G.I. that was popular in the 1940's. Its usually accompanied with a drawing of a man looking over a wall.
- Each door Ulrich checks has the exact same graffiti on it. It says "The Killer".
- This is the first and only episode X.A.N.A. uses a text message.
Ultimatum was released in book form. The book was only released in French.