Eiri Kurahashi, a college art student assisting in his family's antique shop, discovers a Venetian glass showing the reflection of a young girl named Cossette d'Auvergne living within it. This sparks an intense infatuation, driving him to obsessively sketch her image. One night, he communicates directly with Cossette, learning she was murdered in the 18th century by the artist Marcello Orlando, who trapped her soul in the glass. She reveals her freedom requires a man to willingly atone for Marcello's sins through a "Pact of Blood."

Agreeing to the pact, Eiri endures escalating torment as Cossette manipulates his professed love. Wounds manifest on his body, and he experiences hallucinations transporting him to surreal, nightmarish realms. During this ordeal, he discovers he is Marcello's reincarnation, intensifying Cossette's vengeful motivations. Despite the suffering, he remains committed, refusing to repeat Marcello's violence against her. Friends and acquaintances—Shoko Mataki, Yuu Saiga, Michiru Yajiri, and the psychic Zenshinni of Shakado—notice his deterioration and attempt interventions, largely ineffective against the supernatural curse.

The situation escalates when a second manifestation of Cossette, born from Marcello's idealized portrait, emerges. This possessive version demands Eiri as Marcello's vessel. Rejecting her, Eiri creates his own portrait of Cossette using his blood as paint. This act destroys the illusionary Cossette and breaks the curse, severing his connection to Marcello's legacy. In the aftermath, the original Cossette's soul is implied to transfer into Shoko, symbolizing a rebirth. Eiri is freed from the torment, resuming his life with the ordeal concluded.