Isamu doesn't stand out, and everyone calls him a "side character" behind his back. Even his passion for running doesn't get him anywhere on the track team, so he spends his nights languishing at a low-paying part-time job. It feels like everything he's holding in is going to boil over, but when people start getting infected with a mysterious illness that turns them into monsters when sexually aroused, he will have to learn how to control the urges he has pent up inside in order to survive.