Based on the adult manga by Sayu Ayuma.
Keita has been serving Himeko for a very long time, as a childhood friend and as a servant. On the evening before her birthday though, she confesses to having a fiancé, and asks his opinion on it. It turns out she meant a promise he forgot, but he finds a way to make up for it. Yamato finally confessed to his long time crush Nadeshiko, and she said yes! But now that they've been dating, he's scared of defiling her with his lewd thoughts and desires. When she offers him to stay the night after a study session, she makes sure to let him know that she's not as innocent as she appears.