Happiness

A high school friend of Sae Bom, Detective Jung Yi Hyun, informs her that Lee Jong Tae, a member of the Special Operations Unit, is connected to a recent murder. The victim, who was given a substance by Lee Jong Tae's friend, died after the friend bit his neck. Ignoring Yi Hyun's caution, Sae Bom pursues Lee Jong Tae as she strives to earn points for a public rental apartment.

Sae Bom becomes a homeowner after years of effort, only to discover she may have contracted a mysterious illness that causes violent behavior and biting. As she and Seung Young face pressure from reporters offering money for the story, Yi Hyun and Tae Seok from the CDCH work to uncover the origins of the disease.

Sae Bom suspects Tae Seok of keeping secrets, which unsettles her. She and Yi Hyun settle into a quieter life in their new apartment, striving to adjust despite lingering discomfort. Late at night, they are disturbed by persistent knocking sounds coming from the unit above.

The apartment complex is sealed off after Min Ji's mental condition deteriorates due to the effects of the drug Next, forcing all residents to remain inside for a week. Sae Bom and Yi Hyun take steps to isolate a suspect and shelter external individuals, while Yi Hyun and Tae Seok uncover that the Next dealer resides in Building 101.

All apartments in the complex are infected except for Building 101, which becomes isolated as phone lines and internet are cut. Sae Bom and Yi Hyun deal with rising tensions between renters and owners, both inside and outside the building, while struggling with dwindling food supplies and the difficult decision to venture outside.

Returning from a mission to obtain food, the group struggles to convince others of what they witnessed, leading to growing distrust and hidden tensions among the survivors. At the same time, a personal trainer trapped in the fitness center undergoes a disturbing transformation, instilling fear throughout the building.

An infected individual enters Building 101, reigniting fears of an outbreak. Lieutenant Colonel Han arrives with information, but his intentions are questioned by Sae Bom and Yi Hyun. Meanwhile, a resident disappears, and residents begin to form closer bonds with those they trust.

A power outage forces residents to confront their fears as signs of infection appear in Building 101, sparking suspicion and tension. As the lockdown continues, Tae Seok considers a critical course of action, believing Sae Bom may hold the key to a cure.

Residents of Building 101 quarantine Sae Bom and Yi Hyun, suspecting they are infected, and debate how to handle the situation as food and water run low. Tensions rise over how to secure supplies, with the possibility of taking from others if outside help is unavailable. Meanwhile, Sae Bom and Yi Hyun work to identify who else in the building may be infected.

Kim Se Hoon, a resident on the 15th floor, suddenly vanishes, prompting Sae Bom and Yi Hyun to investigate his disappearance and uncover his secrets. The building's other residents descend on his apartment to claim his stored supplies, causing tension and conflict. At the same time, Yi Hyun struggles with his own hidden illness and tries to keep it a secret from Sae Bom.

Sae Bom and Yi Hyun find themselves in peril as resident representative Oh Ju Hyung and other residents collude with Andrew. Tae Seok attempts to rescue Sae Bom after learning of the threat she faces, while Yi Hyun, still concealing his infection, tries to protect her. Their efforts lead to a confrontation with a group of infected individuals.

Sae Bom comes to realize that true happiness lies not in a place but in being with the right person, Yi Hyun, and resolves to return to him despite the dangers she faces.