Hyeong-woo is forced to be quarantined with her sisters-in-law for 21 days due to the new virus. Hyeong-woo, who remains with his sister-in-law is shaken by the temptation of prohibition. Eventually, the virus spreads and becomes the beginning of human extinction. However, rumors are circulating that sharing forbidden love can amplify the energy and defeat the new virus, which they cannot help but believe. Their own sex, which began like that, makes them forget even the fear of the virus and makes them unable to break up.