At the Factory, the gang is waiting for a late Yumi to go ahead with a mission in Sector Five. In the scanner room, Ulrich wonders where she might be, concerned, but Odd makes a teasing remark that she might be spending time with William. This results in a jealous "Hmm?" from Ulrich. Odd adds on "I said maybe." At this time, Yumi is seen taking the sewer passage to the Factory, hurrying. She arrives to the factory just as Jeremie sends the others to Lyoko. He tells them to wait for her while he sends her to meet them.
In the Forest Sector, Ulrich asks Yumi where she was. When she gives him a vague answer, saying she "had something to do," he impulsively accuses her of being with William. She responds by asking if he ever gets tired of being jealous and saying it is none of his business. A small argument breaks out between the two with Yumi telling Ulrich to believe whatever he wants and to be a jerk if he wants to and Ulrich says, "You must think I'm really dumb."
Several Kankrelats then arrive and they battle with the creatures, winning easily. The Scyphozoa then arrives on the scene unnoticed by Ulrich and Odd who are distracted by the Kankrelats, and approaches Yumi, coming up to her, targeting her this time instead of Aelita. The Scyphozoa then attacks Yumi. When suddenly alerted to the monster's presence by Jeremie, Ulrich quickly frees Yumi and the monsters flee. Confused as to why X.A.N.A. had the Scyphozoa attack and go after Yumi instead of Aelita, its main target, Jeremie decides to abort the mission.
He starts with Aelita, Odd and Yumi, but only manages to bring back the first two. Worried, he tries again with Ulrich and Yumi, but only Ulrich returns to Earth. He realizes he cannot devirtualize Yumi. He sends Yumi to a tower while he attempts to figure out the problem. It is then learned that picture day is today and anyone who fails to show up without a solid reason will be suspended on one day. Odd jokes that he'll take the picture for Yumi. Ulrich actually agrees, but says "You? You stink. But I know a really great actress."
At the school, Ulrich asks Sissi to pose as Yumi and that he'll do "almost" anything in return. Sissi agrees, but wants nothing in return. A half hour later, she comes out looking somewhat like Yumi but in a style that looks nothing like Yumi. Ulrich then decides to go get one of Yumi's dresses for her to wear. At the Ishiyama house, Ulrich sneaks into the house to Yumi's room (Yumi's parents, Takeho and Akiko can be heard arguing in the background). Ulrich eventually finds a dress in Yumi's dresser.
At the factory, Jeremie runs a scan of Yumi and finds out that the Scyphozoa stole her DNA sequence, which is needed to materialize her. Aelita has an idea on how to bring Yumi back to Earth and has Jeremie send her to Lyoko, though she refuses to reveal her plan to him. Once she reaches the tower, she activates a process that will transfer her DNA sequence to Yumi, therefore allowing her to come back to Earth. Jeremie tries to protest to this, but Aelita insists Yumi's place is on Earth and that if anyone should remain on Lyoko, it should be her. She then cuts off the signal with him. Jeremie's analysis of the process then reveals that if the DNA transfer goes through, X.A.N.A. will be given free access to take Aelita's memories at will. He then calls Ulrich to help stop Aelita.
Back at Yumi's house, Jeremie's phone call is heard by Akiko. She looks in Yumi's room, but some quick thinking by Ulrich has her believing that Yumi just forgot her cell phone. Once she leaves, Ulrich climbs out of the window. Outside, he meets up with Odd, who has been playing a racing game on his Game Boy. They give the dress to Sissi and head to the factory.
At the class picture, Sissi tries to blend in with the crowd while posing as Yumi. William almost immediately notices that she isn't Yumi, which catches the attention of the rest of the class. All of the commotion alerts the principal, who instantly notices his daughter once he comes to the scene. Sissi says that she has a good reason for what she's doing just as the photographer captures the scene.
On Lyoko, Aelita has nearly finished transferring her DNA sequence over to Yumi. Odd and Ulrich arrive in time to stop her and explain X.A.N.A.'s plan. Jeremie has another idea; since X.A.N.A. stole the DNA sequence, it must then be located somewhere in Carthage. He also warns Yumi that if she is devirtualised without her DNA sequence, like Aelita, she'll be gone for good.
Once they all arrive, they have three minutes to stop the countdown. The switch can be seen at the end of a path across a canyon. They all realize it's a trap, but Odd takes off anyway to get to the key. It turns out that the ceiling is booby-trapped and unsuccessfully attempts to crush Odd as he runs along the platform, being saved by Ulrich. They manage to make it halfway across, but a misstep by Ulrich forces Odd to sacrifice himself, being devirtualized. Ulrich soon reaches the switch and shuts off the timer. They all take the elevator to the access panel.
While Aelita searches for the DNA sequence, Ulrich apologizes to Yumi for their earlier argument and admits that it was none of his business what she had been doing. Yumi tells him, "That's true", before she says she's going to tell him anyway. She was buying tickets to a Subsonics concert, but was having a hard time due to a bad internet connection. The Mantas soon begin to attack as Aelita manages to find the DNA sequence and quickly begins to send it to Jeremie. Yumi begins to tell Ulrich how happy she is to go back to Earth just before she is devirtualized by a Manta. Unsure if Yumi might be gone for good, Ulrich angrily kills the Manta as Aelita asks Jeremie if he restored Yumi's DNA sequence in time for her to be saved. Jeremie tells them to hope for the best and Aelita and Ulrich head back out of the sector.
In the scanner room, it is shown that Yumi has been successfully and safely devirtualized, just out cold on the floor, as Ulrich rushes over to her and holds her in his arms. Yumi wakes up and looks up at Ulrich, who smiles at her and she smiles back at him.
Back at the school, Sissi has taken the heat for Yumi's absence, but assures them she got Yumi out of a punishment. As a gesture of gratitude, Yumi gives Sissi the tickets she bought so she can go with Ulrich. Odd is suspicious of Sissi's selflessness. Aelita theorizes that Sissi might actually be a good person deep down, but Jeremie bluntly says "or she's just crazy about Ulrich." Odd thinks the whole situation is a mess and the episode ends.
- The original French name for this episode is Chaînon manquant.
- First episode where the Scyphozoa attacks someone other than Aelita.
- Only time X.A.N.A. tries to steal Aelita's memory without the Scyphozoa.
- Second time Yumi is at risk of disappearing forever if she ran out of life points.
- Third time Ulrich asked Sissi for a favor.
- This episode shows that Aelita can transfer her DNA codes to someone who doesn't own any on Lyoko.
- The room with a booby-trapped ceiling in Sector Five that the Lyoko Warriors traveled through in this episode would later make a reappearance in Nobody in Particular.
- At the beginning of the episode when Jeremie brought Aelita and Odd back to Earth (also tried to bring back Yumi, but failed), he didn't use the Code: Earth program to devirtualize Aelita, when he should have considering that Aelita is still linked to the Supercomputer (until The Key) and she should not have been able to return to the real world with Jeremie using the normal materialization program.
- When Odd and Ulrich are running through the room in Sector Five, Jeremie tells them that they only have one minute left to trip the key when the Factory Interface shows that they still have a good two minutes.
|Code Lyoko: Season 2|
|New Order | Uncharted Territory | Exploration | A Great Day | Mister Pück | Saint Valentine's Day | Final Mix | Missing Link | The Chips Are Down | Marabounta | Common Interest | Temptation | A Bad Turn | Attack of the Zombies | Ultimatum | A Fine Mess | X.A.N.A.'s Kiss | Vertigo | Cold War | Déjà Vu | Tip-Top Shape | Is Anybody Out There? | Franz Hopper | Contact | Revelation | The Key|
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