ALL HELL BREAKS LOOSE! Basara Toujou is hesitant about his father's sudden decision to re-marry, and yet can't deny it comes with a couple perks -- two adorable little sisters. But Basara's new family isn't all it appears to be, and when his father leaves on an abrupt business trip, Basara learns that one of his new little sisters, Mio, is actually heir apparent to the Demon Lord, while the other sister, Maria, is Mio's succubus attendant. Realizing that his sisters are running for their lives from demons, Basara offers to help in any way he can, but ends up getting more than he bargained for in a magical pact with Mio. Surviving demonic attacks is one thing, but can the pair endure the pact's unique, lustful "side effects"...?