At the start of the series, the King dies of smallpox. Chief State Councillor Lord Cho Hak-ju decides to hide the King's death until his daughter, Queen Consort Cho, produces a son. Such a son would have a more legitimate claim to the throne than Crown Prince Lee Chang, whose mother was a concubine. As a result, the King is inoculated with the resurrection plant and subsequently becomes a monster at night, restrained with chains and fed with the bodies of court servants. Refused permission to visit his father, the Prince leaves in disguise to search for Lee Seung-hui, the doctor who last treated the King. Shocked by what he finds in the process of investigating the King's illness, he heads to the Southern province of Gyeongsang with his loyal bodyguard Mu-yeong to search for more answers. At Lee Seung-hui's clinic outside the city of Dongnae, physician's assistant Seo-bi cares for dozens of patients, but is running out of food to feed them. Yeong-shin, one of the patients, makes a stew for the people from what he says is deer meat. However, the meat is later revealed to be from the cadaver of someone bitten by the King and all the patients quickly turn into zombies.

The next day, the magistrate of Dongnae and nephew of Lord Cho Hak-ju, Cho Beom-pal, disregards Seo-bi and Yeong-shin's advice to cut off the head of each cadaver, leading to a night of chaos when the dead awaken. When the Prince takes control and orders that the bodies of the undead be destroyed, the yangbans secretly flee on the only boat, carrying their belongings and the corpse of a precious son. Soldiers of the royal guard from Hanyang confront the Prince, resulting in the slaughter of dozens of civilians he was trying to help. Subsequently, the Prince goes to Sangju to seek help from Lord Ahn Hyeon, his mentor. Investigating a village that seems to be oddly well-fed in such dire times, he discovers the fate of the missing boat; the precious son became a monster, the yangbans were killed or drowned, and the boat was looted after stranding itself. Awakened from his retreat by his own alarm network, Lord Ahn Hyeon arrives with troops who know how to deal with the zombie crisis. The season ends with Queen Cho taking the Regency of the vacant throne, decreeing the blockade of the South, while Lord Cho Hak-ju comes to Gyeongsang's gate with the Army and the zombie King. On the other side, Lord Ahn Hyeon and the Prince fortify Sangju against the zombies while Seo-bi and Cho Beom-pal explore the Frozen Valley, learning the origins of the zombie disease.

In Hanyang, the Queen has gathered many pregnant women at Naeseonjae, her private residence. Mu-yeong's wife is there as well, taken as a hostage, to force Mu-yeong to spy on the Prince. The pregnant women are killed when their delivery is not as expected. As a result, the Special Forces Commander becomes suspicious and searches Naeseonjae. He finds the corpses of seven delivered women. The baby girls were strangled, but the baby boys were only stillborn. At Sangju, it is discovered that the zombies are slowed not by sunlight, but by temperature. Since the weather gets colder, the situation becomes worse after another night of chaos. The Prince decides to break into the border fortress with a limited squad. They are successful, but fall into a trap. Lord Ahn Hyeon is shot to death, while The Prince is put in the presence of the zombie King, and must cut his head off to save his own life. But there was a trap inside the trap; when Lord Cho Hak-ju turns back to Hanyang, Lord Ahn Hyeon appears as a resurrected zombie. Wearing his commanding flag, he bites Lord Cho Hak-ju, before being killed himself, proving the existence of zombies and the good faith of the Prince. The blockade army allies with The Prince.

The next day, Lord Cho Hak-ju's corpse disappears. He has been taken by Mu-yeong, helped by Seo-bi and Cho Beom-pal. They flee towards Hanyang, but along the way Mu-yeong is killed. At the Lord Cho Hak-ju's residence, Seo-bi discovers a cure; a full immersion in water forces away the worm-like entities causing the zombification. At the same time, the Queen pretends to give birth, presenting Mu-yeong's wife's newborn as her own. She poisons her recovered father who discovered the lie, and organizes a zombie research lab beneath her Palace. While the Queen prepares a large proscription, the Royal Guards are lured out of the Palace, allowing the Crown Prince to conquer the place. But the Queen practices a scorched earth policy and orders to release the zombies from the research lab, and chaos ensues. However, the plan of closing the gates of the inner Palace is not sufficient, and the Queen is eaten. The last few survivors escape to the Rear Garden and break the ice of the pond. The zombies are stopped by the water, while the bitten but not yet transformed people are saved from being zombified.

Seven years later, the baby is crowned King, while Seo-bi, Yeong-shin and the former Crown Prince investigate the Northern Provinces. Shortly after their arrival to a seemingly empty village, a mysterious woman named Ashin appears inside a barn, and stands next to the undead kept inside wooden boxes for some nefarious purpose.

Set before the events of the first season, the episode delves into Ashin's origin story and her experiences growing into a scorned woman that turned against the Kingdom of Joseon. During her childhood, Ashin lived in the Northern village of Seongjeoyain with her ailing mother, and her father, Ta Hab who was one of the Jurchen people living in Joseon and also the head of their settlement. Tensions were high between Joseon, the central kingdom, and the Jurchen border tribes of Pajeowi. The Pajeowi had gathered their army at the basin of the Pajero River.

When fifteen of Pajeowi Jurchen men enter the forbidden area known as Pyesa-gun to illegally gather wild ginseng, they all die unexpectedly. It is revealed that Cho Beom-il, a high-ranking member of the Haewon Cho clan, orchestrated the deaths of the Jurchen men. The Deputy Commander of the Chupajin group, Min Chi-rok was tasked to investigate the deaths of the Jurchen. After discovering Cho's involvement in the killings at Pyesa-gun, he travels to the Seongjeoyain village to conduct damage control and prevent a conflict against the Kingdom by the Pajeowi. Later, Min Chi-rok spreads the message that the deaths were due to a tiger attack but some villagers remained skeptical.

During a meeting with Ta Hab, the Commander requests him to travel to the bordering Pajeowi tribe and spread the message, that the attack was caused by a tiger who killed the men. Ta Hab agreed to the mission as he had pledged his loyalty to Joseon and hoping his actions will grant Seongjeoyain governmental representation, improving the lives of its people. This was later revealed to be a ruse conspired by the Commander, to strategically protect the interests of the Kingdom.

On a false hunt for the alleged tiger, the Commander encounters some Pajeowi soldiers and informs the group, that the attackers of the Pajeowi men were killed by the villagers of Seongjeoyain. This act later diverts the Pajeowi 's hostility towards the village of Seongjeoyain. Afterwards, the Pajeowi military led by Ai Da Gan, attacks Seongjeoyain in the middle of the night, massacring the inhabitants. Prior to the attack on the village, Ashin finds the resurrection plant in a hidden cave shrine and reads the ancient mural to understand its properties and effects, to tend to her terminally ill mother. Upon returning to the village, she finds that it has been burned to the ground and all the villagers killed. Unbeknownst to Ashin, her father was taken hostage by the Pajeowi military.

Being the sole survivor of the massacre, Ashin goes to the Commander and begs for a chance to avenge her family. He places her in a military settlement post, where she begins to perform odd jobs and gets sexually abused by the soldiers, while training in the woodlands. Years later, she was told by the Commander to spy on the military camp of Pajeowi. While infiltrating the camp, Ashin finds her father held as a prisoner, with his limbs amputated. Ta Hab begs his daughter to kill him to end his misery. Ashin complies and sets the camp on fire before going back to the settlement. Prior to her return to the military settlement, the Commander was summoned to return to the South to deal with the war against the Japanese forces, leaving his Second-in-command and soldiers behind. Soon after, Ashin reads the records inside the Commander's hut, implicating the Commander in framing the Seongjeoyain village for the Jurchen deaths' at Pyesa-gun, leading to the eventual massacre of her village.

Ashin prepares to take revenge against the Kingdom of Joseon. After night falls, she kills several Joseon soldiers as they sleep and uses the resurrection plant to bring them back as zombies. While the zombies rampage around the camp, Ashin kills any soldiers, with her bow and arrows, who manage to escape or lock themselves beyond the zombies' reach. Using the last soldier as bait, she attracted the zombies together and then burns them, effectively killing all the soldiers and zombies in the camp. Ashin is then seen dragging a giant sack back to her village which looks normal. She then enters her old house which was filled with her family and some villagers. She was awakened to zombie snarls which brings her to the present where the village is burned down and the house is filled with zombies chained to the back of the house. It is revealed that after the massacre of her village, she had converted her family and some villagers into zombies. Ashin had been feeding them animals for years but felt that they deserved better food and fed them a kidnapped soldier she had carried with her in the sack. She vowed to destroy Joseon and the Jurchen to avenge them and joined them when she is done.

Ashin leaves to meet Lee Seung-hui, the Joseon King's physician at the border in Uiji. She sells him the resurrection plant, setting in motion, the events that would later occur in the "Battle of Unpo Wetland" and the tragedy that would come to pass at Dongnae. After exiting the city, Ashin walks into an open field, and later witnesses Ai Da Gan and his Pajeowi brethren riding horses in the distance. The Pajeowi riders approach Ashin and she shoots an arrow without hesitation.