Echoes is the thirtieth and final episode of Season 4 and the ninety-fifth episode and series finale of Code Lyoko. It is later followed by the fifth season, which is a short reboot titled Code Lyoko Evolution.
In the cafeteria at breakfast, Odd, Ulrich, Aelita, and Jeremie are all in a somber mood as they eat. Sissi arrives at their table to insult them, and secretly places a "mini GPS" chip on Ulrich's jacket before walking away. Yumi then arrives at the table, in a noticeably cheerier mood compared to the others, and reveals that today is the day the group will shut down the Supercomputer, ending their adventures on Lyoko. As the group enters the manhole in the park, Sissi tracks Ulrich from her dorm room.
At the Factory, the group arrives at the Supercomputer Room, where Yumi gives a short speech about starting a new life without Lyoko. Yumi's speech is then followed by a montage commemorating her many battles in the virtual world. However, as Jeremie moves to shut the Supercomputer down, another montage is shown, summarizing the events of the series in chronological order. Jeremie then hesitates, and suddenly calls for a vote to decide whether or not to shut down the Supercomputer. Yumi is the only to vote "yes", much to her frustration, and so the group returns to school.
Meanwhile, in her dorm, Sissi learns that the group has gone to the Factory, and plans to go there herself. Later that day, Jeremie confronts Yumi in the courtyard, and reveals why he does not want to shut down the Supercomputer: his friendships with the other Lyoko Warriors is directly linked to Lyoko, and he feels that once the Supercomputer is turned off, their friendship as a group will end as well. However, after another montage celebrating the strong bond the group has grown to have, Yumi assures Jeremie that their friendship is "too strong to shut down".
In gym class, Sissi is absent, and it is revealed that she has skipped class to investigate the Factory. While Jim gives a lesson on rhythmic gymnastics, Ulrich asks Odd why he voted against shutting down the Supercomputer, and Odd admits that he will miss the thrill of fighting on Lyoko. After a montage of Odd's many battles, Jim injures himself attempting to demonstrate a gymnastic routine. When Odd and Ulrich laugh at him, they are both sent to the principal's office.
As Sissi searches the Factory, she eventually finds her way into the Lab. After turning on the interface, she discovers a video diary made by Jeremie that documents all of X.A.N.A.'s attacks, and begins watching. On campus, on their way to the principal's office, Ulrich and Odd continue their conversation, and Odd asks Ulrich why he voted "no" as well. After a montage of Ulrich's various battles, Ulrich reveals that he enjoyed the satisfaction of "being a hero" that Lyoko provided for him.
Meanwhile, after class lets out, William asks Yumi about the Supercomputer, and she tells him that, unlike her, the others are sentimental about the Lyoko and are still emotionally unprepared for life without it. He then uses their shared opinion of shutting down the Supercomputer as a way of making a flirtatious advance on her, which she quickly turns down and walks away.
In the Lab, Sissi continues watching Jeremie's diary, and has a hard time processing what she learns. As Jeremie and Aelita wait for Odd and Ulrich to be let out of the principle's office, Aelita admits that she voted "no" because she feels that the Supercomputer is keeping her father's memory alive. After a montage reliving Aelita's battles on Lyoko, as well as her first trip to the virtual world with her father, Aelita realizes that it is time to finally move on with her life. As Jeremie and Aelita agree that they are looking forward to more "carefree" lives without the dangers of the virtual world, Sissi confronts them. She reveals all of the things she learned in the Lab and threatens to tell her father.
While Mr. Delmas reprimands Ulrich and Odd for disrespecting Jim, Sissi storms into his office and recounts what she knows about X.A.N.A. and Lyoko. Jeremie and Aelita arrive in the office as Mr. Delmas, believing she is ill, decides to take her to the Infirmary. Afterward, Yumi and Ulrich meet in the courtyard, where he informs her that Jeremie is performing a return in time to erase Sissi's memory. The two then talk about the Supercomputer and, after a montage dedicated to their many romantic moments, Yumi assures Ulrich that they can still have good times together even without Lyoko. They hold hands as he asks her if she promises that and she says yes as the two smile at each other.
The return to the past brings the group back to breakfast that morning. When Sissi arrives at the table, Ulrich stops her from planting the tracking device on him. He then offers her friendship with him and the rest of the group and the others second his offer. Sissi, initially stunned, happily accepts and leaves the table on good terms with everyone in the group, including Yumi, who has just arrived. Yumi then asks the other if they are now ready to shut down the Supercomputer, and they all respond positively.
In the Supercomputer Room, the Lyoko Warriors hold another vote, with results of which are unanimously "yes". Jeremie then shuts off the Supercomputer, and the group exits the Factory. In the park, Jim finds the group coming out of the manhole, and asks what they are doing, to which Odd responds that they have just "closed the book on a unforgettable, adventure-filled life of heroism and danger." This makes Jim reflect on his own teenage years, but as usual, he'd rather not talk about it.
There is then a short montage of the five Lyoko Warriors waving goodbye to the audience. The series ends with a shot of the Supercomputer Room floor, which flashes brightly.
- The original French name for this episode is Souvenirs, which translates to Memories in English.
- This episode is considered an epilogue to the original four seasons of the Code Lyoko franchise, which would eventually be followed by Code Lyoko Evolution (set one year later), which is considered the fifth season.
- This is the only episode in the series in which no one is virtualized onto Lyoko, nor is there any X.A.N.A. attack, as the rogue multi-agent system was finally "destroyed."
- This is the only episode to not feature any cutscenes whatsoever.
- This is also the only episode in which the fourth wall is broken.
- In this episode, Sissi finally discovers what the Lyoko Warriors have been up to, and terrifyingly recalls some of X.A.N.A.'s dangerous attacks from the episodes:
- When Sissi is in the lab and is listening to the video diary that created, Jeremie, in the recordings, makes references and allusions to these following episodes: