The Testament of Sister New Devil

Thanks to his father's sudden remarriage, Tojo Basara finds himself with two beautiful new step-sisters: Naruse Mio and Maria.

After learning that Mio's foster parents were murdered by powerful demons hunting Mio because of her lineage, Basara reveals himself as a member of the hero clan--an enemy of the demons--and vows to protect Mio as a member of his family.

Basara's childhood friend Nonaka Yuuki, a fellow member of the hero clan, warns him not to get involved with Naruse Mio. She claims that Mio will bring great disaster, but Basara proclaims that he will protect Mio as a girl on his own.

Basara suffers a deep wound in the battle against the white-masked man after Mio's power. Mio regrets getting Basara involved, and challenges the white-masked man to battle on her own.

With the battle against the white-masked man settled, they returned to their bawdy daily life. Basara wakes up with his hand groping Maria's butt, he runs into a naked Mio when he enters the bathroom, and Yuki sneaks into the bath with him.

Basara goes on a date with Yuki to the shopping center. When the two of them are overwhelmed by a clerk's sales talk, Mio stops following to rescue them.

Basara leads Mio to higher peaks as he pursues pleasure with the use of syrup. With their master-servant pact reaching a new level, they both bring their full might to face the ones trying to eliminate Mio: Takashi and Kurumi.

Ten days pass since the battle with Takashi and others. Yuki supposedly returned to the hero's village, but when Basara returns home, he finds Yuki in an apron!?

Basara and Yuki begin the ceremony to form their pact. Clad in a bustier and garter belt, Yuki was raring to go for it. But when she refused to forge the pact normally she fell victim to the same succubus curse as Mio!

Mio and Basara are captured following Maria's betrayal! Lars attemps to barter for Mio's release, but his own life is in danger when negotiations go sideways.

Basara is driven by the desire to protect his family. That desire gives him strength, and even as Zolgia pushes him to the edge, he rises without breaking. However, even Basara eventually runs out of strength.

Everyone is trapped inside the body of the demon realm's most dangerous creature: a beast capable of multiplying until it swallows the entire world. The only option for Basara and the others is to destroy the creature's vital core.

Basara finds himself in a compromising situation with the school nurse, but was it all a dream?!