Odd oversleeps after playing video games all night and asks Aelita to think of an excuse for him so he doesn't have to go to gym class. Aelita reluctantly agrees and tells Jim that Odd is in the infirmary. However, when Jim injures himself demonstrating the high jump, he goes to the infirmary and assumes that Odd has already been sent back to his room, but Yolanda says that Odd wasn't in the Infirmary this morning. Jim deduces that Aelita has lied and Odd has truanted. Both students are sent to the principal's office and receive four hours of detention. Afterward, Odd is upset with Aelita for not thinking of a better excuse while Aelita is upset with Odd for getting her in trouble. The two friends quarrel and each forms a grudge against the other.
At lunch, Jeremie makes plans to explore the Digital Sea that night to check the sphere they saw last time despite the animosity within the group. However, when the Lyoko Warriors head over to the Factory after dark, Aelita and Odd are spotted entering the building by Nicholas and Herb, who are fishing in the Seine River not far from the Factory, so they decide to follow them inside to find out what Odd and Aelita are doing there.
Odd, Aelita, Yumi, and Ulrich are sent to Sector Five, where they all board the Skidbladnir. As Odd and Aelita's argument begins to wear on the others' patience, the ship disembarks, and they travel to the other virtual world they had discovered in their last expedition.
The group arrives at the new virtual world, which is revealed to be an exact copy of the Forest Sector, and they disembark from the Skid. Tired and infuriated by Odd and Aelita's pettiness and bickering, Ulrich and Yumi go on a reconnaissance mission around the Replika and leave Odd and Aelita to guard the Skid. Meanwhile, in the Factory, Nicholas and Herb begin searching aimlessly, and come across a hatch that leads to the lab and they become very curious when they see a light coming from it.
On the Replika, Odd and Aelita finally settle their differences just before two Megatanks arrive and begin attacking the Skid. Aelita uses her Creativity to conjure a massive barrier to protect the ship while Jeremie redirects a significant amount of the Supercomputer's energy to the Skid to reinforce its shields.
Jeremie tells Yumi and Ulrich to return to the Skid to help defend it. Suddenly he hears a noise made by Nicolas and Herb not far away from the lab and decides to check the Factory's security cameras only to discover Herb and Nicholas that are about to see the lab. Because Jeremie has just rerouted power from the Supercomputer, it does not have enough energy to perform a return to the past, and Jeremie is forced to leave the lab and distract Nicholas and Herb from discovering the Supercomputer.
Meanwhile, William arrives on the Replika and helps the Megatanks destroy Aelita's barrier. A battle ensues, but Odd quickly runs out of laser arrows, rendering him defenseless. In the Factory, Jeremie succeeds in luring Nicholas and Herb away from the lab. He then returns to the lab and shuts down the elevator. Assuming Odd and Aelita are somewhere below, Nicholas and Herb wait for them in front of the elevator on the Factory's main floor.
Meanwhile, on the Replika, Aelita defeats one of the Megatanks and Odd is knocked off the platform by William, forcing Aelita to devirtualize him before he falls into the Digital Sea. Yumi and Ulrich then arrive at the Skid and defeat the last Megatank. William, now outnumbered, flees into the Digital Sea. In the lab, Odd devises a plan to throw off Nicholas and Herb, but requires Aelita to return to Earth.
Ulrich devirtualizes Aelita, and Yumi pilots the Skid back to Lyoko. At the Factory, Odd tells Aelita that in order to kick Nicolas and Herb from the factory they both need to go up to the main floor. When the elevator arrives, Nicholas and Herb find them kissing inside. Assuming Odd and Aelita use the Factory as a private place to be intimate, Nicholas and Herb leave.
Later, all five Lyoko Warriors convene in the lab, where Jeremie is worried; the existence of Replikas means there are other supercomputers in the world infected by X.A.N.A., and that to rid the world of X.A.N.A., Jeremie must find a way to destroy these supercomputers. The others, however, are optimistic and reassure Jeremie that they will defeat X.A.N.A. someday. The next day, Nicolas and Herb tell Sissi about what they saw yesterday but Aelita and Odd dispel rumors that they are dating by staging a theatrical argument in the campus courtyard.
- The original French name for this episode is Réplika, which is exactly the same as the English title, but with an accent mark on the "e".
- One of Jeremie's farewell clips in Echoes comes from this episode; the others come from Bragging Rights, A Lack of Goodwill, and A Space Oddity.
- Yumi pilots the Skid in this episode, marking the first of only three instances in the original series where someone other than Aelita does so; the other two times occur when Odd pilots the Skid in Bragging Rights and Cold Sweat.
- When Jeremie finds Herb and Nicholas on the security camera and hears them talking, Herb does not appear to have a mouth.