Code Geass: The Miraculous Birthday

Code Geass: The Miraculous Birthday (コードギアス キセキの誕生日) is a half-hour long picture drama.

Plot
Four months have passed since Lelouch's death. Rivalz has taken control of the Student Council, which also includes Nina, Kallen, and Gino. Ashford Academy prepares for yet another school festival. Lelouch and his friends reunite for one last adventure to help prepare for the festival while narrated by Jeremiah Gottwald.

While preparing for the festival, C.C. interrupts and claims to Lelouch that she is his girlfriend and that they are engaged. An annoyed Lelouch demands why she told his friends their secret. She replies to be honest. Suzaku suggests that the wedding take place at Kururugi Shrine and Nunnally agrees. Lelouch is still disgusted with this, but Suzaku tells him to promise him never to be mean to girls again.

However, they soon find themsleves hostages of Neo-Chinese Federation nationalists as they take over the school. Lelouch concocts a plan to defeat the terrorists, during which some of the characters start noticing inconsistencies with the canon.

Meanwhile, Suzaku and Kallen fail to defeat the Eunuch leader (who appears to be a fusion of a human and a Gun-Ru knightmare frame) leading the terrorists. Lelouch then suspects he has a Geass as he and others move in to face the Eunuch at his headquarters (The same chapel from Nunnally Held Hostage). C.C. reveals that the Eunuch is armed with a scientifically unexplained artifact collected by Charles zi Britannia called the Thousand Man Silver Vase. After losing Nunnally and C.C.'s souls to it as well.

Lelouch finds a weakness in the Vase's power and changes into his Zero costume. While Rolo stops the Eunuch's sense of time with his Geass, Lelouch uses a subliminal Geass order for the Ashford Student Body to wear Zero's mask. When the Eunuch orders "Zero's" soul to be taken by the Vase, it overloads and releases everyone. The Eunuch tries to reassert his control by using a gun but Suzaku uses his "Spinzaku" kick to beat him down. In the end the student council succeeds and Lelouch bids his final farewell to his beloved friends. It is revealed that Lelouch, using his Geass via the afterlife, has put everyone in a trance and created the events of the OVA as a final gift to them, having not gotten the chance in life. He apologizes to Rivalz for not revealing the Zero Requiem to him, encourages Nina to continue using her power, thanks Kallen for returning to Ashford as he had requested while telling her never to lose hope in each other, encourages Nunnally to use her seeing eyes for good, and tells Suzaku to use his status as Zero for everyone. To everyone's shock, he then fades away along with C.C., Rolo, and Shirley. The remaining members realize this "miracle" is only possible annually on that particular day, December 5th... Lelouch's birthday.

After the illusion ends, Kallen chastises Lelouch for engraving her love for him in her mind permanently while working with Rakshata on the Guren similar to how Suzaku did with Lloyd; Nina thanks Shirley for being her friend while Gino plays around in the background; Nunnally, with Kaguya and Tianzi, wonders if she and Rolo could ever have a sibling relationship like they did with Lelouch; and Suzaku decides he can still never forgive Lelouch but bids him Happy Birthday regardless, his status as alive now known to the Student Council.

Quote

 * Conversation between Rolo and Nunnally
 * "Oh, Rolo?" -Nunnally
 * "What, Nunnally?" -Rolo
 * "I'm big brother's little sister, right?" -Nunnally
 * "Yeah. And I'm his little brother." -Rolo
 * "So does that make me your big sister? Or am I your little sister?" -Nunnally
 * "Er..." -Rolo
 * "Did you cry when mother died? Or were you angry, perhaps? I can't remember your face at all..." -Nunnally