This is just a general summary. I’ll post one that’s much more in depth once all the crazy shenanigans die down.
EXIII is basically volume 8 chapter 2.5.
It opens by referencing the renovations Zest made to the Toujou house following chapter 2 of volume 8, as well as Maria and Zests relationship since splitting up the housework. Then it shifts to Basara, Mio, and Yuki’s first day back to school after returning from the demon realm. While Mio and Yuki are eating lunch with Aikawa and Chika, Basara goes to the infirmary looking for Chisato. He ends up finding Nanao in the infirmary and she’s experiencing her first period.
Basara then gives Nanao a belly rub until she goes to sleep due to medication that Chisato gave her. After Basara places Nanao on the bed, He leaves the infirmary only to find Chisato standing in the hallway against the wall.
It then talks about how it’s been half a month since Basara and Chisato last saw each other on Christmas Eve. Then it mentions how attractive Chisato is today, and how her clothes are different from normal. (She’s wearing the “virgin killer” sweater like in the illustration)
It then mentions how attractive Chisato is and how when she takes of her white coat she acts as a woman for Basara, and how that’s a sign for him to act as a man instead of a student. She then says “longtime no see” with a smile and the two hug. They eventually start kissing in the hallway and Basara gropes her butt. Since they could use the infirmary, they use the student guidance room instead. After Chisato locks the door, she sits down in a chair, and spreads her legs (like in the illustration). She then asks Basara what he thinks of her clothes, and she mentions how she bought them for him. Basara then notices she isn’t wearing a bra and begins to fondle her chest. The breast fondling progresses into paizuri, and then it describes all of the times Chisato has helped Basara since the beginning of the series, and how his sexual experiences with her are what helped him deepen his bonds with the girls in the demon realm. Then there’s talk about the advice that she gave Basara which helped him with his decision to kill Belphegor. The scene ends with the two of them naked and she eventually invites him to go to the hot spring with her.
It then transitions to after school. Basara and the girls have a tradition where they share with each other how their day went. Mio and Yuki talk about how Aikawa and Chika want to come over. This causes Maria and Zest to remember how they used Mio’s friends against her back when they were working with Zolgear.
Zest curse activates and then Mio’s curse activates shortly after. Of course, Basara has to step in to relieve the girls. It starts out with the scene from the illustration. (there’s no penetration) Kurumi’s vibrating underarm pads from volume 9 make their debut. It eventually transitions to Mio getting her sensitive place rubbed, but Basara makes sure not to put in any fingers. Mio gets relieved and end of EXIII.