Jeremie tells Aelita that it is her 'first birthday', explaining that it's been one year since they first discovered her and Lyoko -- the "birth" of their friendship. Jim can be heard yelling at Odd and Ulrich in the background, asking if they're asleep. Realizing that Jim might yell at him too, Jeremie stops his work and gets to bed.
Jeremie and Odd arrive at the Factory and find the lift is not working. They take an alternate route through the machine rooms, but X.A.N.A. has taken control of the machines, and uses them to try to kill the two. They run for the Boiler Room, which is on the way to the lab. Jeremie suspects the boilers are also part of the trap, but since they have no choice, they run for it, avoiding injury as the boilers explode behind them.
Later, Ulrich and Yumi go to the Factory, not realising that Sissi, suspicious about their activities, has followed them. She confronts them on the Factory Floor and gets into the Lift with them, which unexpectedly closes and traps them inside. The lift falls at huge speed before finally stopping in the shaft a long way below the Lab. Ulrich's arm is broken in the fall, and Sissi is knocked out cold. Yumi is unhurt, and she climbs out of the lift and up the shaft to get to the Lab.
Yumi makes it up to the Lab, and Jeremie sends her to Lyoko too.
In the lift, Sissi has awoken, and is frightened. Ulrich comforts her, and she binds his broken arm in a sling.
Unfortunately, water has started to leak into the lift shaft through a crack in the wall, and enters the lift, gradually filling it up. Ulrich tries to hold the ceiling hatch closed, but finds it difficult with his broken arm and cannot stop the flow.
On Lyoko, Odd and Yumi get Aelita to the tower just in time, as Ulrich and Sissi are about to drown, and Jeremie activates a time reversion.
Back in the past, Jeremie, Odd, Ulrich, and Yumi are in Jeremie's dormitory, crowded around Jeremie's computer. Aelita is on the screen, wondering what this is about. They reveal a cake with a single candle on it to celebrate Aelita's birthday.
Sissi, Herb, and Nicholas sneak to the open door and watch. Ulrich notices her and she tries to hide, but, remembering her kindness before the time reversion, he smiles at her. Nicholas comments on how weird it is to celebrate their computer's birthday, but Sissi, pleased by Ulrich's reaction, declares it a good idea, and tells Herb and Nicholas to get a cake for tomorrow to celebrate the birthday of her make-up kit.
- The original French name of this episode is Piège.
- The group celebrates the one-year anniversary of when they first met Aelita in the two-part prequel, X.A.N.A. Awakens
- This is the only episode from the original series that X.A.N.A. has attacked with the machines in the factory.