With the increasing number of people getting infected, Han Tae-seok along with his military team are struggling to determine whether to preserve these people, or to kill them off. Han Tae-seok initially tries to keep the mad person disease a secret, but with more cases progressing and people getting panicked, it is revealed to the public. The question of humanity and whether to view those infected with mad person disease as human beings forms a psychological battle for Sae-bom, Yi-hyun and Tae-seok. Jung Yi-hyun conflicts with building resident Oh Joo-hyung who seems to encourage destructive behavior in the building. In an effort to protect the residents in his building, Jung Yi-hyun and Yoon Sae-bom establishes quarantine rules, while securing the building from the outside with the help of his Captain, Kim Jung-guk.

This begins the outbreak in the building. This assumption is later confirmed when a resident member who consumed the drug, secretly sold to her in the building's gym, ultimately became infected with the mad person disease and succumbed to the illness. Through Jung Yi-hyun's detective work, he encounters a diseased person who he believes became this way due to a failed pneumonia drug called Next, that has been widely circulated in order to promote attentiveness and strength. Yoon Sae-bom meets many interesting and strange residents, forming a close attachment to her neighbor's young daughter. Class discrimination is evident where the upper floor resident Oh Yeon-ok struggles to be the apartment representative while keeping everyone in check. 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. 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. 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. Some of those who contracted the disease are ultimately able to return to their former selves temporarily. The disease is not airborne, but spreads through scratches and bites, with Yoon Sae-bom seemingly immune to the disease. It is revealed that the symptoms of mad person disease is first accompanied with unabated thirst, followed by the pupil turning white, and then the biting of human necks. The facility houses many of the 'patients' where they are kept under observation in order to understand and cure the disease. In the struggle to subdue the trainee, she is scratched in the process, leading her to meet Han Tae-seok at a research facility. Yoon Sae-bom encounters a trainee at the Special Operations Unit who is affected by mad person disease.

The South Korean Military and Police forces are trying to contain the spread of Rita and Next through extensive investigation and authoritative quarantine measures, while civil rights groups protest against them, believing the infected are still capable of normal human interaction. Those infected by Rita experience short bouts of insanity and bloodlust, before completely regressing into a zombie-like state. Happiness takes place in the near future, where the release of a failed Covid-19 treatment drug 'Next' has caused a worldwide pandemic known as the Rita Virus, a.k.a 'mad person disease'.

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