After his father's death, Basara Toujou becomes the step-brother of Mio and Maria Naruse. However, he soon finds out that they are actually a demon lord and a succubus, and he is now bound to protect them.

As Basara learns to navigate his new life with his devilish step-sisters, he also attracts the attention of other demons and heroes who seek to use Mio's power for their own purposes.