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". Those infected by Rita 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 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.