Intrusion

The gang has planned a routine data gathering mission in the Cortex. However, Ulrich opts out of the mission when he finds out that Yumi wrote some negative things about him. While in the Cortex being attacked by Ninjas, Yumi can't stop thinking about Ulrich. She gets herself devirtualized on purpose just to go talk to him and work things out. The two soon reconcile.
Meanwhile, a Ninja, not defeated in the fight in the Cortex, follows the Megapod back to the Skid, then hangs onto the Skid all the way back to the hangar in Sector Five. William ends up being left alone to fend off the Ninja, which is heading for the Core of Lyoko itself. Ulrich is called in and easily destroys the Ninja with his sabers, but it plants a sinister-looking device on the bottom of a platform in Sector Five before it disappears.