The Testament of Sister New Devil

Thanks to his father's sudden remarriage, Toujou Basara finds himself with two beautiful new step-sisters. Their names are Naruse Mio and Maria. Mio wakes him up in the morning, while Maria stands in the kitchen wearing a suggestive outfit...

As daughter of the former demon lord, Mio's foster parents were slain by powerful demons after her life. Still seeking to slay her, 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 warns him not to get involved with Naruse Mio any longer. She is a fellow member of the hero clan, and the one charged with observing 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. Meanwhile, in order to protect Mio, Basara resolves to use the "forbbiden art" he sealed away after the events of his past. What was the tragedy that visited the hero village and Basara?

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. The master-servant pact lying at the heart of everything. Accoring to Takigawa, his classmate and the white-masked man, the pact between Basara and Mio holds another secret. Meanwhile, Basara's childhood friend Yuki invites him out on a date...

Basara goes on a date with Yuki. Unaccostomed 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. Although the battle ended with reconciliation, Yuki returned to the hero's village. ...or she was supposed to, 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 she refused to forge the pact normally, and suffered the same succubus curse as Mio.

Mio has been captured by Zolgia following Maria's betrayal. Basara too is captured by Lars and brought to Zolgia's mansion. Lars' goal is to exchange Basara for Mio. He explains that he's only doing his duty as Mio's observer, but negotiations fail, and Zest slays him.

Strength as a human, the power to rend evil--With the power inherited from his father, Toujou Jin, Basara faces off against Zolgia. Basara is driven by the desire to protect his precious 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 facting that all too powerful might. His mind in despair, he's unable to escape from Zolgia's dominion...

As Yuki fends off any enemy approaching the core, Basara, Mio and Zest soon fight against a horde of Zest's clones. The group arrives at the core and Zest offers herself as bait for Basara to kill the beast. In doing so, however, Basara protects Zest and disables the vital core with his Banishing Shift, thus killing the beast. As the girls recover, Basara witnesses Yahiro/Lars executing Zolgear. Back at their home, Zest places her loyalty to Basara. Greatly opposed to the fact of someone else stealing Mio's virginity, Basara and Mio engage in lovemaking until Basara stops mid-way, apologizing for forcing himself unto Mio. As Basara and Mio were about to kiss once more, Yuki interferes and steals Basara's lips and is accompanied by Zest. As Mio and Yuki argue over Basara, the latter runs to Maria and asks her to help him appease the two. Adding to the uproar, Maria lunges at Basara and kisses him. Shella and Zest were then put under the care of the demons from the moderate-faction. Meanwhile, word of Zolgear's death eventually reaches Demon Lord Leohart as he faces Jin Tōjō in his throne.