Yoon Sae-bom encounters a trainee at the Special Operations Unit who is infected by mad person disease. In the struggle to subdue the trainee, she is scratched, leading her to meet Han Tae-seok at a research facility. The facility houses many of the "patients" who are kept under observation in order to understand and cure the disease. It is revealed that the symptoms of mad person disease are first accompanied by unabated thirst, followed by the pupil turning white, and then thirst for human blood. The disease is not airborne, but spreads through scratches and bites, with Yoon Sae-bom seemingly immune to the disease. Some of those who contracted the disease is ultimately able to return to their former selves temporarily. Yoon Sae-bom makes a deal with Han Tae-seok which leads her to secure an apartment in a newly constructed building, however in order to do so, she must be married. She asks her high school friend Jung Yi-hyun, a former star baseball player but now a police detective, and he agrees to be her fake husband. The building is luxurious and highly stratified, with the public rentals consisting of the lower five floors, and the upper floors for those who own their apartment. Class discrimination is evident where the upper floor resident Oh Yeon-ok struggles to be the apartment representative while keeping everyone in check. Yoon Sae-bom meets many interesting and strange residents, forming a close attachment to her neighbor's young daughter.