The episode begins with an image of a comet approaching Earth. Meanwhile, the Lyoko Warriors, conduct a search for Franz Hopper and William on the network, and Jeremie uploads a new program to the Skid that will allow it to better detect bio-signals. It is then shown that only Aelita, Odd, and Ulrich are aboard the Skid. Yumi, along with the rest of her science class, is busy helping fifth graders create a scale model of the solar system on the school track.
In the Skid, Aelita detects a faint trace of DNA in the area, and she begins traveling to its position. Meanwhile, in the lab, Jeremie watches a news segment on the comet VG724, which is passing close to Earth later that day. On campus, Yumi is paired with her younger brother Hiroki and his friend Johnny, and together they are tasked with creating a scale model of Earth and the Moon and plotting their orbits but they don't take their tasks seriously making Yumi even more angrier as she had preferred going to Lyoko.
In the lab, the Superscan suddenly finds an activated tower in the Desert Sector, and Jeremie reluctantly orders the Skid to abandon the DNA signal and return to Lyoko. Meanwhile, it's revealed that X.A.N.A. has possessed a military-grade satellite and uses it to fire a laser at the comet, causing it to break apart and change directions, creating in a massive meteor shower that begins falling towards Earth. After receiving a call from Jeremie, Yumi sneaks away from campus and begins traveling to the Factory.
After watching a suspicious emergency news report about the comet breaking apart, Jeremie researches VG724 and discovers the meteor shower is going to strike in the vicinity of the Factory. He deduces that most of the most of the comet debris will burn in the Earth's atmosphere, but there is also one huge stone that will be capable of destroying the Supercomputer and everything else in the area. Jeremie figures out that X.A.N.A.'s plan is to "kill two birds with one stone": destroy the Supercomputer with the meteor shower and trap Aelita in the Digital Sea so that she can be captured. Aelita tries to convince Jeremie to get out of the Factory immediately, but he refuses, opting to stay to perform a return in time at the right moment before the meteorite can destroy everything. Jeremie then calls Yumi, who is in the park, and tells her about the meteor shower. Fearing for safety of her brother, Yumi disobeys Jeremie and returns to campus.
As the Skid approaches the Hub, it is trapped in a firewall, forcing Aelita and Jeremie to work out a way to crack it. On campus, Yumi tries to warn Ms. Hertz and Jim about the meteor shower, but Jim believes those were just a bunch of fireworks she saw. It is only when smaller meteors begin entering the atmosphere and become visible in the sky that Ms. Hertz takes Yumi's warning seriously and tells Jim that they need to evacuate the school. Ms. Hertz convinces Mr. Delmas of the approaching danger and the fact they are obliged to ensure students' safety, to which he agrees to a calm and quiet evacuation of the school.
Back in the Digital Sea. Jeremie and Aelita manage to breach the firewall by finding its weak spot and free the Skid. Jeremie worries that the Skid will not make it back to Lyoko in time before the meteor shower, and decides to hack the military satellite and attempt to change the meteors' course away from Earth.
Kadic Academy is evacuated under the pretense of a fire drill as Jeremie gains access to the satellite. He then calls Yumi, who is still on campus, and asks her to hook his laptop up to the antenna on the roof of the Science Building so that he may link directly to the satellite. However, as she slips away from a crowd of students, she is spotted by Hiroki and Johnny, who decide to follow her.
Meanwhile, Aelita and the others arrive back on Lyoko, and use the Skid to travel to the activated tower. Once she, Odd, and Ulrich disembark from the ship, they are confronted by a single Kankrelat. But once they reach the tower close enough, the Kankrelat retreats and three Tarantulas along with William take its place, forcing the group to retreat to nearby rocks. In the Science Building, Yumi reaches a locked rooftop door, and Hiroki and Johnny catch up to her. Yumi is furious when she sees them and tells them to leave immediately. Hiroki instead demands to know what Yumi is doing, but does not believe her when she tells him about the meteor shower. However, when a small meteorite crashes on campus and shakes the building, Hiroki and Johnny concede and pick the lock on the door for Yumi.
On Lyoko, Ulrich, Odd, and Aelita are vastly outnumbered and are forced to retreat. Meanwhile, on the roof of the Science Building, Yumi and Johnny work together to hard-wire Jeremie's laptop to the antenna. However, the brunt of the shower is already too close to Earth to revise its course, and the largest meteor enters the atmosphere directly above the school. On Lyoko, Aelita notices the Tarantulas haven't fired at her even once, and realizes X.A.N.A. wants to capture her alive and destroy the Supercomputer, but if she is on Earth, X.A.N.A. cannot accomplish either of those goals. So, Aelita orders Odd to devirtualize her to which he reluctantly agrees. With Aelita back on the Earth, X.A.N.A. is forced to use the satellite to blow up the large meteor, saving Aelita, the Supercomputer, and the rest of the Earth.
Admitting defeat, X.A.N.A. deactivates the tower manually and rematerializes the Tarantulas while William retreats back to the network. Jeremie then activates a return to the past. After all that, the Lyoko Warriors meet in Jeremie's dorm room, where Yumi announces that she, Hiroki, and Johnny received the highest grade in the class for their models of Earth and the Moon. The episode ends with Yumi half-jokingly worrying about her privacy now that she knows that Hiroki can pick locks and locking her room is no use.
- The original French name for this episode is Corps céleste, which translates to "Celestial Body" in English.
- The comet's ID was Comet VG724.
- The top-left screen of Jeremie's computer confirms that this episode is set on May 18th, 2007.
- The woman news reporter that was seen in Big Bug marks her second and final appearance in this episode.
- This episode also features a search engine seemingly modeled after Kiwi.
- This episode marks the second time where X.A.N.A. takes possession of a satellite; the first being a military one with an extremely strong and destructive laser beam in "Satellite".
- This episode marks the only appearance of the Fire Wall in the Digital Sea.
- This episode reveals that both Hiroki and Johnny are capable of picking locks.
- The Lyoko Warriors (correctly) deduce that X.A.N.A. still wants Aelita alive. However, up until this point they have only witnessed him trying to throw her into the Digital Sea, so it seems unlikely they would come to this conclusion.
- At the beginning of the episode, Ms. Hertz refers to Yumi's class as ninth graders. However, as of the beginning of the third season, Yumi is in tenth grade and the other Lyoko Warriors are in ninth.
- Despite its only purpose, to take the real William's place at Kadic until the Lyoko Warriors find a way to free him from X.A.N.A.'s clutches, the William Clone is not present in this episode for unknown reasons.
The original episode summary can be found here.