|Spoiler warning: Plot or ending details follow.|
The Lost Christmas is the twelfth episode of the Guilty Crown anime. It first aired on January 13, 2012.
Shuichiro teleports to the crystal tower but not before giving Segai the information he wants as thanks for his service. Meanwhile, Shu finally recognize who Gai is but he tells Shu to rescue Inori before the youngster, who Gai calls "Death", teleports away. As Shu follows them through a portal, "Death" tells Shu he still doesn't remember his past and causes him to remember.
In the past, Shu and his big sister, Mana, rescued a boy who she named Triton, after finding him on the beach. Triton, who is actually Gai, began living with the Oumas and looked up to Shu who was then a brave boy. After experiencing his flashback, Shu finds himself in the crystal tower where Shuichiro has Inori binded and "Death" preceding a "wedding ceremony" to transfer Mana's soul, contained in a pod, into Inori's body so that Shuichiro and Mana will give birth to a new race and spreads the virus worldwide. Shu gets another flashback where Mana telling him she loves him and wants to marry him. But in truth, Mana was infected with the Apocalypse Virus which was making her insane.
When Gai learned about it, Mana threatened him not to tell Shu. Back in the present, Shu tries to stop them but crystal-like plants controlled by Mana hold him. However, Gai and Ayase's Steiner arrives thanks to Segai. As Steiner is destroyed, Gai helps Shu remember how the Lost Christmas incident began. On that day ten years ago, Gai informed Shu to meet him at a church to tell him the truth about Mana. However, Mana came instead and tricked Gai into shooting himself.
When Shu arrives at the scene, he is disturbed on what happened and rejects Mana's offer to marry her and start a new race together by calling her a monster. Hurt by what Shu said, Mana goes berserk and spreads the Apocalypse Virus over Tokyo before coming to her senses and destroys her body. Feeling responsible for not helping Mana, Gai vows to become stronger while a traumatized Shu made himself forget about Mana and Gai. Now remembering his past, Shu helps Gai extract his Void, a gun that forces out other people's Voids. As Shu rescues Inori, Gai heads to the pod to free Mana's soul, Shuichiro tries to stop him only to be killed by "Death". Gai reaches for the pod but is stabbed by the crystals protecting it.
Knowing he won't live much longer and wishing to accompany Mana in her last moments, Gai begs Shu to stab Mana's pod through his body with Inori's sword which Shu tearfully does. As the tower crumbles and Gai and Mana disappears together, Shu and Inori float to safety and mourn for Gai.