Nightmare of Nunnally is a reboot of Lelouch of the Rebellion, focusing on Nunnally and her contract with Nemo, the relationship with her friend Alice, and just what she wants to accomplish with her power. While many of the plot points remain similar to Lelouch of the Rebellion's timeline, there are many new characters and differentiated characters. Geass has changed, now able to affect the physical in addition to the mental. Some are born with the ability to tap into Geass without a contract, and some have been altered with CC's cells to grant them a form of Geass.
With Britannia oppressing the world, Nunnally vi Britannia only wishes for a peaceful, gentle world. After the apparent death of her beloved brother, Lelouch, Nunnally enters into a contract with a part of the Witch, named Nemo, and with its power regains sight and movement, as well as a powerful Knightmare. Fighting against the Britannians in revenge for her brother's death, she eventually learns that nothing is as it seems, and that the power she has goes far beyond her wildest dreams. She has the power to create the world she dreams of… but at what cost?