So we left off in Vol. 2 that Mai has discovered that she is in love with Mr. Saionji. She still wonders whether or not if he is truly in love with her or just using her.
In this next part of the story, Saionji decides to pop-up on Mai at her apartment and will stay with her for a while. During his stay, Mai wants to do something nice for him buy making sugared bread crusts because his childhood friend, Kei Fujiwara, told he likes sweets. In between the nice gestures, there are so many spicy and heated scenes (which if you follow @theprettyotaku on twitter you get the exclusive previews of all the scenes in a thread per volume).
Kei, in the midst of a coincidence, decides to invite Mai to his birthday party in other to thank her for sharing the bread crusts with him. When Saionji finds out that she has been invited to the party he calls him, and they discuss what his true intentions with Mai are. He didn't reveal anything to.
The night of the party, Mai shows up in a beautiful dress that she borrowed. She gets big-eyed and excited about the party with all the food and expenses. The woman who joined them on the hot spring retreat was there as well. She judged Mai and made her a spectacle. She pressured her into sharing what she was capable of buying Kei, for his birthday. She carved him an aquarium wood for his collection. In return, everyone laughed at her and made her feel out of place. She then runs off.
Saionji was able to catch her, in return he compliments her difference in financial status and shares a passionate (spicyyyyy) moment with her outside on the balcony. When they go back inside to the party, they separate and Mai ends up bumping into Kei again. He goes to show her his aquarium collection and the wood that she had given him, that is now placed in a new tank. That's when everything starts to get interesting. He pins her against one of the tanks and asks her if she has ever thought about him.