Based on the adult manga by Arima Jin.

Akira and her lover Naoto get distracted by sex in the bath. As a result, Naoto contracts a cold. Akira is worried about him, but when she comes to check on him, he distracts her with sex and they begin to mess around, only for him to overexert himself. No need to worry, though... Akira takes care of him! Then, Akira has to work on Naotos birthday, but before she can bring home her double pay, she accidentally downs a leftover alcoholic drink. Naoto is called in to bring her home, but his birthday has already ended, and Akira is so upset she begins to cry. Theres only one thing to do, really: celebrate right there, with plenty of sex!

(Source: H-Moe)