The X.A.N.A super-calculator has mounted another attack against our planet. This time, it has taken control of little Milly’s teddy bear. Milly is a student at the same junior high school as our heroes. Not only does the teddy bear come to life and become very aggressive but he grows to be as big as a building before our very eyes.

Jeremy manages to get himself grounded for the whole school holiday period. He wants to be there in case X.A.N.A, the super-calculator, should decide to launch an attack over the break. Sissi, who suspects Jeremy has secret activities, works things out so that she is also at school during the holidays.

Sissi uses Ulrich’s diary against him so that he will be forced to pay attention to her. The diary contains details of Ulrich’s secret activities and of the existence of Lyokô. She takes advantage of a school trip to the swimming pool to attack.

X.A.N.A has created a computer virus and sent it out on the Internet. All the city’s databases have been damaged. Although the problems this causes are quickly solved without major incident, the railway control centre has been hit very hard and can no longer stop two of its trains, which are on a collision course.

X.A.N.A doesn’t let a single detail of life at the school get by him. The headmaster has decided to make extensions to the gymnasium and the super-calculator wastes no time before turning the bulldozers from the construction zone into weapons.