The Mitsurugi household was given the important task of guarding the demon gate by the emperor.
Sora Mitsurugi, who was the heir of the family, was called on to take the trial ceremony at the age of thirteen.
He has to overcome the trial in order to learn the phantom blade style that was passed down the Mitsurugi household from generation to generation.
The other seven pupils of the same age who had taken the trial have all passed. And now only Sora remained.

His father, brother, fiancee, and a pair of siblings who have watched over him were all looking, as Sora’s trial ceremony solemnly began… (Novelupdates)

Set between the first and second seasons of the anime.

2017 a.t.b. The "Black Rebellion", the large-scale revolt against Britannia caused by Zero, ends in failure in Area 11. However, the influence of the Black Rebellion as a sign of opposition against the increasingly powerful superpower was strong. In addition to the major powers opposing Britannia like the Chinese Federation and E.U., acts of anti-Britannia terrorism began to occur with greater frequncy in the countries under Britannia´s rule, and a new period of resistance arose.

At the same time, the Britannian homeland begins to nip the buds of resistance with its overwhelming military strength. Within its forces, there is an anti-terrorist unit formed by Princess Marrybell mel Britannia, the "Glinda Knights". Deploying ace pilots and a great number of experimental Knightmare Frames on the Caerleon-class floating airship "Grandberry", the Glinda Knights are given the search-and-destroy mission of suppressing the terrorism that occurs in all parts of the world.

In the midst of such a chaotic situation, a boy and girl with the same face and the same name "Oz" encounter each other by chance in a small country in the Middle East.

Note: The manga story occurs alongside the Code Geass - Soubou no Oz SIDE: Orpheus photo story that is published in Hobby Japan magazine.