Code Geass: Boukoku no Akito 4 – Nikushimi no Kioku Kara Episode 1

While W-0 is away, Leila and the remaining crew at base find themselves surrounded by Britannian Knightmares. Piloting the Vercingetorix, Shin Hyuuga leads the enemy charge, having usurped the position of Grand Master in the Knights of St. Michael. With her core members unavailable, it's up to Leila to protect her comrades, but this may be much tougher than she realizes: a spy lurking among them is finally ready to reveal himself.

Left with no choice but to surrender, Leila arranges a meeting with Shin in order to ensure the safety of her squad. But as negotiations begin to fall apart, Akito and an unexpected ally return in the nick of time to turn things around.

(Source: MAL Rewrite)