Geassed operators collapse the outer wall of the city, and the battle begins.
Let's back up. Two people have learned about Zero's power:
Suzaku has been told by V.V., a mysterious boy.
Villetta Nu has recovered from her amnesia and infiltrated the Knights' command center.
Suzaku attacks Zero, but is trapped by a field that disables Lancelot.
The Knights are using the school as a command center. When Villetta causes chaos, the student council move to free Suzaku from the field.
With everyone else out, V.V. comes for Nunnaly.