Setting

Happiness takes place in the near future, where the release of a failed treatment drug "Next" has caused a worldwide pandemic known as the Lytta Virus, a.k.a. "mad person disease". Those infected by Lytta experience short bouts of insanity and bloodlust, before completely regressing into a zombie-like state. The South Korean Military and Police forces are trying to contain the spread of Lytta 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.

Plot

Yoon Sae-bom (Han Hyo-joo) 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 (Jo Woo-jin) 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 are 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 (Park Hyung-sik), 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 (Bae Hae-sun) 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.

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, which has been widely circulated in order to promote attentiveness and strength. 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. This begins the outbreak 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 (Lee Jun-hyeok). Jung Yi-hyun conflicts with building resident Oh Joo-hyung (Baek Hyun-jin) who seems to encourage destructive behavior in the building. 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.

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. 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 ordered the complete quarantine of the new building complex where Yoon Sae-bom and Jung Yi-hyun lives, offering them a chance to leave the building before it becomes completely sealed to anyone going in or out of the complex area, however they decline the offer. Yoon Sae-bom's blood contains antibodies that could hold the cure for the disease, which Han Tae-seok seeks to exploit. It is revealed that Han Tae-seok's pregnant wife was bitten by the president of the pharmaceutical company that supplied Next, and ultimately became infected with the disease. His search for the cure is in part motivated to find a cure for his wife and to save his unborn child.

Main

• Han Hyo-joo as Yoon Sae-bom

A member of a Special Operation Unit police squad (KP-SOU) formerly known as KP-SWAT. She is decisive, determined, smart, and does not easily get rattled. She is excited to move into her new apartment, but as soon as moves there she faces a crisis.

• Park Hyung-sik as Jung Yi-hyun

A smart and honest detective who, for a long time, has held romantic feelings for Sae-bom. They graduated from the same high school. He struggles to protect Sae-bom and other people.

• Jo Woo-jin as Han Tae-seok

A lieutenant colonel and belongs to the health service command. He holds a key to the infectious disease outbreak.

Supporting

Special appearances

• Lee Kyu-hyung as Lee Seung-young^[24] (ep. 1–3, 8 and 12)
• Baek Joo-hee as Park Min-ji (ep. 2–3)
• Lee Seung-joon as Kim Dae-yoon, a brigadier general (ep. 12)

Part 1

Part 2

Part 3