Following the events of their trip to Okinawa, Saitama University graduate students Shinya Yukimura and Ayame Himuro have failed to obtain the data necessary to scientifically prove their love for each other. Unable to replicate the exact conditions of the trip, the two stubborn scientists decide to seek help from elsewhere in the university—the Biological Sciences department. Assisted by fellow graduate students and longtime couple Chris Floret and Suiu Fujiwara, Yukimura and Himuro begin to quantify their feelings by measuring their output of oxytocin in various romantic situations. The two scientists soon find that their feelings for each other are nothing compared to that of a mature couple. It is here that Chris poses a question to both Yukimura and Himuro: what will they do if their "affection" is proven not to be love after all? Desperate to find a scientific rationale for the discrepancy, Yukimura puts everything on the line to prove that their "love" is real.

Reito Mizuhara is a medical student at the National Advanced Medical School in Tokyo who contracts a rare, lethal disease called cellular sclerosis. In order to be treated, he is put into cryogenic sleep for five years. Upon awakening, he learns that a year into his slumber, a worldwide pandemic broke out, caused by the novel Male Killer (MK) virus which exclusively afflicts males and has killed all other men on Earth. Fortunately, Reito and four other men were spared during their cryosleep and are now immune to the virus. As the only men left in the world, they are thus tasked with mating with as many women as possible to repopulate the planet. However, Reito is committed to his childhood friend and sole love interest, Elisa Tachibana—only to learn that she went missing three years ago. While the other male survivors adapt accordingly to this new world, Reito instead chooses to pursue Elisa, fight never-ending temptation, and investigate this mysterious virus.