At breakfast, Jeremie announces to Ulrich, Odd, and Aelita that he has found a way to materialize them onto Earth from a Replika in the vicinity of the supercomputer generating that Replika. After breakfast, the group meets with Yumi outside the cafeteria, where Jeremie and Aelita relay the news to her and add that they plan to test Jeremie's idea that night. However, when Ulrich attempts to speak to her, Yumi acts coldly and walks away.
Odd, stunned by what happened, sks what he did to earn such a cold reaction. Ulrich says how he has no idea, but that it began the day before yesterday. Odd reveals how Yumi's birthday was on that date - and Ulrich realizes he forgot about her birthday. Odd brings up how important such days were to Yumi and she'll likely be mad for weeks. Ulrich wishes to immediately go buy her an apology/birthday gift, but realizes he can't since he had detention for a prank he was blamed for by Jim. Odd then offers to buy the gift for Ulrich since he was heading into town to buy a new toy for Kiwi - and the prank Ulrich was punished for was actually done by him and he owed him for it.
That evening, the Lyoko Warriors travel to the Factory, and Odd discreetly gives Ulrich Yumi's wrapped gift, but refuses to say what it is with the genuine assurance that she'll love it. Ulrich decides not to give Yumi the gift right away, and she remains aloof as they are sent to Lyoko and board the Skid. Aelita then pilots the Skid out of Lyoko and into the Digital Sea, where she uses a Hub to get to the Forest Replika.
Jeremie detects an activated tower on the Replika, indicating X.A.N.A. is active somewhere on Earth. Aelita then moors the Skid to a different tower, which Jeremie activates for himself. Jeremie successfully translates Odd and Aelita to Earth while Ulrich and Yumi disembark from the Skid to stand guard.
Odd and Aelita find themselves in the Amazon Jungle, and discover they have retained their Lyoko avatars and powers. Jeremie explains that they are essentially spectres similar to ones X.A.N.A. uses, and reveals that they only have a limited amount of time on Earth. Aelita notices the laboratory containing the supercomputer beyond a cluster of trees, and she and Odd begin walking toward it. Meanwhile, Ulrich is desperately trying to earn Yumi's forgiveness, repeatedly apologizing for forgetting about her birthday. However, Yumi tells him how she has never forgotten his birthday and refuses to accept his apology.
Suddenly, Ulrich and Yumi are confronted by William and three Tarantulas, and a battle ensues. Yumi takes on the monsters while Ulrich engages William. In the jungle, Aelita and Odd manage to infiltrate the laboratory, where they find all the scientists inside are possessed by X.A.N.A. They then find a room containing a vivarium full of robotic spiders before being discovered by one of the possessed scientists, and a fight breaks out.
On the Replika, Ulrich is devirtualized by William as Yumi defeats the last monster. William then manages to steal Yumi's fans and destroy them, leaving her defenseless. As Yumi dodges William's attacks, Aelita and Odd defeat the scientist in the laboratory and find the supercomputer behind a locked door. Aelita uses her Creativity to open the door, but they then discover the supercomputer is guarded by some sort of force field.
Jeremie manages to virtualize a new pair of fans to Yumi, but she is devirtualized by William, who enters Jeremie's tower. Odd is able to destroy the shield protecting the supercomputer just as William enters the X.A.N.A. Code into the tower, severing the Skid from the tower and forcing Odd and Aelita back to the Replika before they can destroy the supercomputer itself. Aelita pilots the Skid out of the Replika under laser fire and returns to Lyoko.
Later, the Lyoko Warriors gather in the Lab, where they discuss the failed mission. As the group leaves the Factory, Ulrich stops Yumi on the bridge and gives her the gift. Yumi finally forgives Ulrich for forgetting her birthday and kisses him on the cheek. leaving him blushing. Aelita looks on in envy, unable to remember when her own birthday is. Jeremie tells her to simply pick a date herself as her birthday. However, Aelita asks Jeremie to pick the date for her. He does so, but won't tell her when the date is - wanting it to be a true surprise for her.
After returning to their dorm, Odd opens of box containing Kiwi's new toy while Ulrich watches, but finds a necklace instead. It is then revealed that Odd accidentally switched the gifts and gave Yumi the box with Kiwi's toy in it instead of the one containing the necklace. Meanwhile, in her room, Yumi unwraps her gift and finds a squeaky rubber bone, much to her confusion.
- The original French name of this episode is Expérience. In Israel, it was called "Laboratory Mouse".
- This episode only aired on television in France; it did not air in the United States for unknown reasons. However, the episode with complete English dub was released on Netflix the same year. It also aired in Australia when Cartoon Network got around to airing seasons 3 and 4. It also aired on Kabillion.
- This episode marks the first time in the series someone other than Aelita travels to the second floor of a tower and inputs a code.
- This episode used the Season 1 opening sequence instead of the Season 4 opening sequence for some reason.
- In the scene where Ulrich lets go of Odd after the bell rings, Milly is seen in the background in her Season 1 - Season 3 clothes instead of her Season 4 clothes.
Episode summary from here.