After ending the battle against Zolgia, Basara regains his peaceful life with his two little sisters, Mio, the daughter of the previous demon lord, and Maria, her little sister who's also a succubus. As Basara pursues the preparations for the sports festival at school, one day he's attacked by a person being manipulated by magic. Recalling the strange gaze he felt at school, a suspicion of his begins to grow...
I'm an adult who matured earlier than Onee!
Ich bin eine Erwachsene,
die früher als Onee gereift ist!
¡Soy una adulta y maduré 
mucho antes que Yuki!
¡Soy una adulta y maduré 
mucho antes que Yuki!
Je grandis plus vite que Yûki 
et je suis plus mature !
Sono un'adulta! Un'adulta che è maturata
 prima della sorellona!
Eu sou uma adulta que amadureceu
 mais cedo que a Onee!
Я взрослая! И расту быстрее сестры!