On August 10, 2010, a.t.b., Japan was conquered by the Holy Empire of Britannia. Seven years later, teenage student Lelouch Lamperouge finds himself pulled into a battle between anti-government terrorists and the Britannian Army in the Shinjuku Ghetto. In the midst of the conflict, a mysterious, green-haired girl saves him from death by granting him "the power of the king", which allows him to compel others to do his bidding.

The Purist faction within the Britannian Army scapegoats Suzaku for Clovis' death, planning to use him as an excuse to rid the army of Honorary Britannians. Lelouch concocts a plan to save his childhood friend, with some help from Kallen and Kaname Ohgi, leader of Kallen's resistance group. Announcing himself publicly as Zero, Lelouch declares it was he who killed Clovis, and uses his power on Jeremiah Gottwald to force him to assist in Suzaku's rescue. In private, Lelouch offers Suzaku the opportunity to turn against Britannia for falsely accusing him, but Suzaku refuses, wishing to change the Empire from within.