The episode begins in Carthage, where Aelita is accessing XANA's data. With the Mantas about to hatch, Jeremie asks Aelita to stop so he can take over. The Mantas then emerge, forcing her to stop. Jeremie brings up the vehicles and everyone hops aboard. While waiting for Jeremie to open up the data stream, Ulrich jumps onto a pursuing Manta and destroys it, but he falls. Odd breaks his fall with his Overboard and everyone escapes the sector. With the data collected, Jeremie thinks he can develop an antivirus for Aelita.
When they get back to Kadic Junior High School, it's late at night. As Jeremie and Aelita talk in a dormitory hallway about Aelita's alias, they are being spied on by Sissi, who records the conversation on her walkman. Meanwhile, in the lab, the computer screen turns on for no reason.
The next morning, on Tuesday, everyone is getting ready for school. Yumi is confronted by her father on why she's out so late. At school, Jeremie tries to cheer up Yumi, telling her it's going to be "a great day". Later in the day, Jeremie is confronted by Sissi, who shows him the taped conversation. She threatens to show her father, the principal, the tape if he didn't tell her what was really going on. In Mrs. Hertz's class, Jeremie tells Odd and Ulrich about Sissi's threat. They are interrupted by Mrs. Hertz, who then drills Odd with questions on photosynthesis.
After class, Ulrich practices Pencak Silat in the gym, hoping to distract Sissi. As Sissi tries to keep up with Ulrich, Odd and Jeremie finger through Sissi's belongings to find the tape. Unfortunately, they are caught red-handed by Herb and Nicholas, who parade them to Sissi. As Sissi walks off, an unexpected time reversion sweeps by, bringing everyone back to Tuesday morning. With a second chance to get at Sissi, Ulrich asks her to join her in practicing Pencak Silat. In class, Odd correctly answers Mrs. Hertz's questions, already knowing the answers from before the time reversion. In the gym, Ulrich tells Sissi that Jeremie and Aelita are crazy, saying," for some strange reason, they think they're virtual people."
At the factory, Jeremie discovers that the data they recovered from Sector 5 allowed XANA to take control of the supercomputer's time reversion feature. Immediately after Jeremie locates the activated tower, another time reversion starts, bringing them back to Tuesday morning again. This time, the gang goes over to the factory immediately after class. Aelita, Yumi, Ulrich, and Odd enter Lyoko's Mountain sector, where they are confronted by a squadron of Hornets. Aelita and Odd jump off the path, landing on another path below while Yumi and Ulrich continue to fight the Hornets.At school, Sissi shows her father, the principal, the tape. The principal then calls Odd's parents to find out if Aelita is really Odd's cousin. On Lyoko, Yumi is devirtualized by the Hornets. Ulrich jumps off the edge to meet up with Aelita and Odd as the Hornets pursue. At school, Principal Delmas finds out Aelita isn't Odd's cousin, but another XANA-controlled time reversion brings everyone back to the start of the day.
This time, XANA possesses Sissi and makes her confront Jeremie. On Lyoko, Aelita has been hiding from the Hornets the whole time, but the others can't help her until Jeremie virtualizes them. Sissi takes Jeremie to the Boiler Room, where she is confronted by Odd and Ulrich. Sissi pulls out a metal pipe and attacks Ulrich. As Odd takes Jeremie back to the Lab, Ulrich and Sissi fight with metal pipes. On Lyoko, Aelita is captured by the Scyphozoa, which begins to drain her memory. Yumi and Odd enter Lyoko, cutting off the Scyphozoa's tentacles and freeing Aelita. Now, the only thing between Aelita and the activated tower was a Tarantula. After a few minutes of fighting, it's destroyed. Aelita deactivates the tower, just moments before Sissi was going to kill Ulrich with her metal pipe. The next day, Jeremie theorizes that jumping back in time increases XANA's power. From now on, they would have to limit time reversions, but unfortunately, it will be difficult.
This episode contains examples of:
- Caught on Tape: Instead of going inside Aelita's room to talk in privacy, Aelita and Jeremie load an entire confession of Aelita's true origins in the hallway while Sissi is spying on them, and records them with her walkman. She uses this in an attempt to get into the gang, and in the second time loop, succeeds in telling her father, who then calls Odds parents....and then everything resets. When the final time rewind occurs, Ulrich steps on the walkman to erase the recording while Sissi also doesn't remember anything due to being Xanafied.
- Déjà Vu: The gang notices this, and because of their collective Ripple-Effect-Proof Memory, they all band together to see what's going on.
- Foreshadowing: Odd getting smarter by learning about photosynthesis (and he oddly found it enjoyable knowing the right answers for once), which directly parallels XANA getting smarter by resetting the time loop.
- Flaw Exploitation: XANA gets smart enough to take control of Sissi in the third loop, the first time he's controlled a Human in the real world, just to stop Jeremie from being this trope to him, as XANA realises that even if he resets time, the gang will try and reset everything back to normal just as quickly, and he can't keep resetting time too quickly, or his plan becomes pointless to act out.
- "Groundhog Day" Loop: The premise of the episode is that XANA keeps resetting time to make the super computer more powerful, as the more it resets, the more time and more calculations it can do to become smarter. This also has the side effect of Odd being uncharacteristically smart about photosynthesis, in the third loop taking over for the teacher and goes into what appears to be great detail about the process.
- Loophole Abuse: Odd uses this to get good at science, while the rest of the gang use it by way of their Ripple-Effect-Proof Memory to stop XANA.
- Nice Job Fixing It, Villain: Thanks to XANA possessing Sissi, it eventually leads to her destroying the evidence she recorded that Aelita is not really Odd's cousin, which would had terrible consequences for the heroes.
- Out-of-Character Moment: Odd uses the time loop to great effect, and for Mundane Utility purposes; to explore science, more specifically Photosynthesis, with the implication that by the third loop, he's giving Mrs. Hertz a run for her money and winds up teaching her class. He surprisingly enjoys the subject and starts reading up on it in the lab.
- Oh, Crap!: The gangs' general reaction to XANA controlling a human; Sissi in this case, something not exhibited in the show before now as something he cannot do directly.
- Running Gag: "It's Tuesday already!"
- Save Scumming: Odd uses this to get really good at learning the process of photosynthesis. Hell, he even studies because he finds knowing stuff cool which he ends later teaching his class. Yumi also starts eating her breakfast more quickly to avoid talking to her parents about her antics at night.
- Sesquipedalian Loquaciousness: Jeremie explaining to Odd how the supercomputer gets smarter with each time loop, and when he asks for a laymans example, Jeremie just says the supercomputer is smart enough to get smarter.
- Shout-Out: To Groundhog Day of course, but the opening scene of each time loop is a more modern interpretation of the opening loop from the film. Phil Connor wakes up to a clock radio talking about the cold weather, while Jeremie wakes up to a digital alarm clock radio that also talks about the sunny weather.
- Time Loop Fatigue: XANA manages to take control of the "Return To The Past" capability of the supercomputer it's trapped in. It ends up restarting the same day over and over again, which drives the heroes who have ripple effect-proof memories, completely bonkers (except Odd, who is actively exploiting it to learn about photosynthesis and get ahead of his one class), as they are forced to live the same day over and over again while trying to figure out how to stop XANA from resetting time. XANA for once, isn't doing this specifically to spite the gang, however - each time reset increases the supercomputer's processing power, which makes XANA more powerful, annoying the heroes was a side benefit of this.
- Time Loop Trap: One of the interesting parts of XANA's plan is that this is an unintended upside to his plan of getting the supercomputer cleverer. Though he later uses it to reset time when the Lyoko Warriors get close to stopping him on two occasions.
- Wham Episode: The Reveal that every use of Return to the Past increases XANA's power. By the end of this episode, XANA has grown strong enough to control people directly. This knowledge leads the heroes to decide to use Return to the Past more sparingly in the future.
