Onsen de Himitsu no Otomari, 241-kaime

If there's one thing middle-aged salarymen Ishida and Kimura love more than each other, it's a good hot spring. These two have carved out a weekend every month to spend a steamy night together soaking in the rejuvenating waters, carrying on a tradition from their youth. Hidden from their friends and colleagues, their love has simmered quietly as the decades passed. Sure, they both agreed either one could call it off anytime, but for twenty years, they've never missed a trip—because the thought of lying in each other's arms warms them more than any hot spring ever could.

(Source: Yen Press)