Orphaned boy Mamoru Ichinose is taken in by the Amagasaki family, where he finds a peaceful new life with his doting adoptive sister Noa, who affectionately calls him "Mamochi." Blessed with rare exorcist talent, his future seems bright—until the succubus queen Rosalia Violetta awakens. In an instant, Rosalia warps the city. Mamoru's memories are erased. A fake sister pleasures him, a fake mother drains him, and at a school-wide "virginity graduation ceremony," he surrenders his virginity, life force, and exorcist power to Rosalia. Drowned in pleasure, his mind rewritten, he becomes her blind, devoted plaything.

Then Noa—the sole holdout against the brainwashing—bursts through warped space. Her voice restores his sanity. Though powerless, Mamoru escapes and, blessed by the angel Pamela, vows to take back his life, his friends, and the sister who never gave up on him.