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.
Now that X.A.N.A. has finally been destroyed, the gang are feeling depressed and nostalgic. Everyone has their own reasons for not wanting to shut down the supercomputer. While Yumi liked being a devoted Lyoko warrior, she is ready to move on with her life without Lyoko. Jeremie does not want to lose the only link to their friendship, and Odd and Ulrich both liked being heroes. Aelita, meanwhile, is afraid to let go of her father and her past. Each reminisce their fond and cherished memories of their heroic lives.
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 calls for a vote to decide whether or not to shut down the Supercomputer. Yumi is the only "yes" voter, 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 friend group will fall apart. However, after another montage celebrating the bond the group shares, 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, and when Odd and Ulrich make fun of 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 principle's office, Ulrich and Odd continue their conversation, and Odd asks Ulrich why he voted "no" as well. After a montage of Ulrich's 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.
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 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 move on. As Jeremie and Aelita agree that they are looking forward to more "carefree" lives, Sissi confronts them about 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 says he does not believe anything Sissi is saying, and opts to take her to the Infirmary instead of taking her seriously. Afterward, Yumi and Ulrich meet in the courtyard, where he informs her that Jeremie is performing a Time Reversion to erase Sissi's memory. The two then talk about the Supercomputer, and after a montage dedicated to their romantic tension, Yumi convinces Ulrich that they can still have good times together without Lyoko.
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, and offers her friendship. The others second his offer, and Sissi, initially stunned, accepts, leaving 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.
- This episode is considered an epilogue to the original four seasons of the Code Lyoko franchise, which would eventually 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 to Lyoko, nor any X.A.N.A. attack as the rouge multi-agent system was finally "destroyed."
- This is the only episode in which the fourth wall is broken.
- In this episode Sissi finally discovers about the Lyoko Warriors, and terrifyingly recalls some of X.A.N.A.'s dangerous attacks to her father from the episodes: