The story revolves around a teenage girl named Aya and her twin brother Aki, members of the wealthy and powerful Mikage family.

On their sixteenth birthday, they are called to gather in their family home to be given a special present. Instead they find themselves the victims of an ancient family ritual. Aya is proven to be the reincarnation of a celestial maiden and upon reaching the age of sixteen her body is able to bring forth a powerful, celestial maiden called Ceres, the tennyo. When she transforms into Ceres, Aya obtains supernatural powers and often forgets what she has done during the transformation, resulting in a double personality. According to the legend, if left to live, the reincarnation of Ceres will bring ruin upon the Mikage family. Aya quickly finds herself assigned a death penalty by her own family. However, she is saved by Suzumi Aogiri, another tennyo descendant, from Kansai and Suzumi's brother-in-law, Yūhi. They take Aya into their home after she is forced to run from the Mikage.

Events escalate as Aya struggles to control Ceres and her brother gains the ability to manifest as "Shiso Mikagi", the ancestor of the Mikage family who stole Ceres' celestial robe (羽衣, hagoromo^?), without which she cannot return to heaven. Aya must also wrestle with affection for Toya, a Mikage employee with no memory of his own past. He is assigned to protect Aki and observe Aya by the leader of the C-project (celestial project). As he carries out his mission, he begins to develop feeling for Aya, and she for him.

As Aya's allies increase, which include two C-Genomers (celestial descendants) like herself and the Aogiri family, and Tōya, she manages to make an agreement with Ceres. Aya promises to retrive Ceres's "hagoromo" in exchange for the celestial maiden's mercy toward her brother and Toya.

Aya's quest to find Ceres's hagoromo wavers as she struggles with the emotional trauma of losing her family, battles the organization carrying out the C-Project, fights off the advances of Shiso Mikagi, and defies the forces that threaten to tear her away from Tōya. Tōya also struggles to find himself and regain his lost memory. In doing so, he learns that he was the organism that was created from the hagoromo or mana and his purpose was to return the mana back to Ceres.