The film opens with a precarious situation surrounding a concubine of the former king, Park Ji-young, who is left without any blood connection to her stepson, the current king, a childless widower known as Sung-won, portrayed by Kim Dong-wook. In her desperation, she devises a plan to have her compliant son ascend to the throne. Unbeknownst to her, the timid prince finds himself falling deeply in love with Hwa-yeon, played by Cho Yeo-jeong, the daughter of an aristocrat. Hwa-yeon, however, has already given her heart to Kwon-yoo, represented by Kim Min-jun, a low-born commoner.
As fate would have it, when Hwa-yeon’s father, Ahn Suk-hwan, is compelled to send her into the royal palace as a concubine for the king, the young lovers attempt to flee. Their plans are thwarted after they share a single night together, leading Hwa-yeon to accept her fate in the palace in exchange for saving Kwon-yoo's life.
Fast forward five years, Hwa-yeon has now become queen, having given birth to a son. Word of the king's ill health reaches Sung-won while he is on a journey, prompting him to return to the palace. During a private moment, he gifts Hwa-yeon a hair stick, revealing his lingering feelings for her. Shortly thereafter, the king succumbs to an enigmatic illness. With the king dead, Hwa-yeon is relegated to a monitored, modest residence while the former concubine positions her son as the figurehead king, declaring herself as Regent and Queen Mother, thus controlling the royal court with an iron grip.
When Hwa-yeon's father, a loyal court minister, attempts to disclose the truth behind the previous king’s death, claiming it was an act of poison, he and any remaining ministers loyal to him are imprisoned for treason. In the midst of this turmoil, Hwa-yeon discovers Kwon-yoo working among the eunuchs in the castle. Though initially thrilled to reconnect, she is met with hostility, as Kwon-yoo has been left embittered by his circumstances, even going so far as to align himself with the Queen Mother. As tensions rise, Kwon-yoo's resentment leads him to obstruct Hwa-yeon's efforts to save her father from execution, ultimately agreeing to assassinate Hwa-yeon and her child under the Queen Mother’s orders.
Despite the dangerous landscape within the palace, Sung-won is entranced by Hwa-yeon, lavishing her with attention. In a moment of jealousy, he takes Geum-ok, Hwa-yeon’s maid, as a minor concubine, hoping to glean information about Hwa-yeon’s personal habits. An intense confrontation occurs when he finds Geum-ok wearing the hair stick he had gifted to Hwa-yeon. Geum-ok, in a moment of desperation to save her own life, reveals Kwon-yoo's past with Hwa-yeon, including suspicions about the parentage of Hwa-yeon's child. This leads to a violent encounter between Hwa-yeon and Sung-won, culminating in Hwa-yeon rejecting his advances and challenging him to become a true king.
Following these events, Kwon-yoo confronts his emotions when he believes he is the father of Hwa-yeon’s child, prompting him to protect her and their son at all costs. Kwon-yoo concocts a plan to poison Sung-won, but finds himself compelled to drink the poison to safeguard Hwa-yeon and their child. As Kwon-yoo writhes from the effects, he reveals the truth about Minister Yoon, who secretly supports the Queen Mother's devious plans.
Hwa-yeon, finding herself imprisoned at the hands of the Queen Mother for treason, learns of the conspiracy against her and her son. Sung-won, horrified at his mother’s confession about her role in the previous king’s death, realizes he has never desired to be king himself. When Kwon-yoo is brought forth to determine the truth behind the poisoning plot, he chooses to lie, implicating the Queen Mother and saving Hwa-yeon from ruin.
In a poignant farewell, Kwon-yoo asks Hwa-yeon to safeguard their son as he faces execution, to which Hwa-yeon heartbreakingly contends that their child belongs only to her, a stark reminder of the sacrifices made.
Ultimately, Sung-won emerges as a true king but does so alongside a grim fate as Hwa-yeon strategically dispatches him. The film concludes with Hwa-yeon donning a smile as she observes her son playing on the throne in the abandoned royal court, a façade that quickly vanishes as the door closes behind her, sealing her fate and the depth of her loss.
