It's her 30th birthday and she has no boyfriend. It's that guy who has transferred to Mihono's company. 10 years ago, they met Ureshino Eisei at a mixer just to make up the numbers, even though they both had partners. They don't make promises, they only meet when it's convenient and have sex - that's all there is to their relationship. For some reason, Mihono hasn't heard from him for 10 years, and she thought they'd never see each other again...!! But Eisei hasn't changed at all since then, and he continues to flirt with Mihono as if nothing had happened...