A South Korean family celebrates the wedding of an astronaut. Then the groom receives a call on his wedding night that he is needed for a reconnaissance flight during which he observes a nuclear test that triggers an earthquake in Korea. Strangely, the quake's epicenter keeps moving towards Seoul. A photographer who was documenting the moving temblor's damage notifies authorities that its cause is a gigantic underground creature making its way toward Seoul. Martial law is declared. As the creature, Yongary, emerges, it begins to wreak havoc on the city. Later, it is seen drinking from a refinery's storage tanks to fuel its flaming breath. Icho, an eight-year-old boy, also notices that it disliked a tank of ammonia when it was ruptured. Il-Woo, the boyfriend of Icho's sister, is a scientist. He recognizes the creature's aversion response and concocts a precipitate of ammonia. The first batch puts Yongary temporarily to sleep, so a stronger form needs to be created. This is taken up in a helicopter to be deployed.