The gang return to the Cortex to get data on the mysterious Ninjas. Ulrich opts out of the mission because he has a karate competition to attend, and Jim won't let him skip it. The others don't fare well against the Ninjas until Ulrich arrives, offering them advice that Jim passed on to him; be unexpected. With the advice given by Ulrich, and a new-and-improved, unusually effective bo staff weapon programmed for Yumi, she manages to defeat the three Ninjas easily and gathers the data. In it they find a video clip of Professor Tyron addressing several humans dressed in ninja-like costumes. Ulrich wins the competition and gives his trophy to Jim, thanking him greatly for his advice.

Ulrich veut rester au lycée en raison d'une compétition scolaire de karaté. Le reste du groupe lui reproche de faire passer ses activités personnelles avant leurs missions sur Lyoko. Sur le Cortex, les héros se préparent à affronter de nouveaux ennemis, les Ninjas, qui attaquent de façon bien plus imprévisible que les monstres habituels.

Los Guerreros de Lyoko necesitan volver sin esperar a Córtex para obtener nueva información de Ikonov. Pero parece una peligrosa misión contra los ágiles y precisas "criaturas negras" que protegen el núcleo. Y sobre todo Ulrich, involucrado en una competencia de karate interescolar, no puede permitirse ir a la misión.