Set in Korea’s Joseon period, a few years after the Japanese invasions of Korea (1592-1598), the first season depicts the story of Crown Prince Lee Chang (Ju Ji-hoon), who becomes embroiled in a coup/political conspiracy and is forced to embark upon a mission to investigate the spread of a mysterious undead plague that has beset the current emperor and the country’s southern provinces. The story starts with a notice on a billboard, claiming the king of Joseon is dead and the crown prince should be crowned immediately as the new king. However, inside the palace, the king is known to be severely sick and has been secretly treated for the past 10 days. No one was allowed to visit the king, not even the crown prince himself. Late at night, the crown prince sneaks out to visit his father, wanting to know the truth. But instead of his father, he encountered the silhouette of a beastly monster. Accompanied by the physician Seo-Bi (Bae Doo-na), the enigmatic Yeong-Shin (Kim Sung-kyu), and his personal guard Moo-Young (Kim Sang-ho), Prince Lee-Chang must prevent the advance of the plague towards his home capital of Hanyang (present-day Seoul) while addressing the sinister coup masterminded by Minister Cho Hak-ju (Ryu Seung-ryong) and his family aimed towards his deposition from the throne.
The second season centers on the advancement of the plague across several provinces. In the lead-up to halting the outbreak, a devastating revelation ensues at Sangju. Upon the arrival of the winter season, the undead hordes now roam freely due to a change in ambient temperature. Fearing the ramifications of an imminent invasion, Prince Lee-Chang attempts to garner support from his allies to stop the undead and reclaim his right to the throne. Unbeknownst to the Prince, another conspiracy for political gain is brewing within the corrupt Haewon Cho Clan. As Queen Consort Cho (Kim Hye-jun ) schemes to steal a male newborn to cement as heir to the royal throne, she further orchestrates a last ditch strategy to use the plague against the citizens of Hanyang, if her former plan fails to succeed. As calamity looms among the living, only one question remains. Can the Prince save his people in time and end the political turmoil at the capital or will the plague mark the end of the Joseon dynasty?