Ancient Korea, 1506. The tyrannical King Yeonsan-gun of Joseon is overthrown by his half-brother Jung-jong, whose reign begins with a blood bath. Over the years, traitors plot against him, sinking the kingdom into chaos. In 1528, frightened rumors come to royal palace, regarding a mysterious creature, known as Monstrum by the peasants.

Yoon So-So works as a travel guide in France. She leads a group of tourists on a package tour. The travelers all have their own stories. San Ma-Roo joined the tour after being dumped by his girlfriend. Kim Gyung-Jae has been dating for the past 10 years. A mysterious man seems to follow Yoon So-So. Jung Yeon-Sung joined the package tour with a partner, but he will not reveal what kind of relationship they are in. Han So-Ran is a web designer and agonizes over staying single or marriage. Oh Gab-Soo is extremely stubborn. Han Book-Ja spent most of her life taking care of her husband. Na Hyun is a woman whom other people cannot guess her age. They all do not want to become involved in each other's personal lives, but, while they travel together, they become closer and develop relationships.

Martial law is declared when a mysterious viral outbreak pushes Korea into a state of emergency. Those on an express train to Busan, a city that has successfully fended off the viral outbreak, must fight for their own survival…

Six different stories of six couples in a particular room in a hotel in Singapore.

As a North Korean sleeper cell agent, Ryu-han infiltrates the South and assumes the role of a village idiot in a rural town. He observes the townsfolk and waits patiently for his mission. One day, after 2 years of playing the role of the village idiot, fellow elite spies Hae-rang, posed as a rock star and Hae-jin, posed as an ordinary student, are dispatched to the same town as Ryu-han. He helps the other two spies settle in and teach them how to adjust in the South. There is a sudden drastic political power shift in the North and all three spies receive an urgent and ultimate mission.