The story of "My Royal Nemesis" begins 300 years ago, during the Joseon era. A red-tailed star appears in the sky, heralding a surge in crimes and disasters. These ominous signs coincide with the promotion of Lady Kang Dan-sim from a humble concubine to a royal consort. The palace deems her a threat to the order and demands her execution. They frame her for attempted murder of a prince and a fellow consort to justify her demise.
Kang defends herself, claiming that her only crime is to survive in a treacherous place like the palace. But she is ultimately forced to drink poison as her shaman, Hwang, secretly performs a ritual with her blood. As she nears death, Kang witnesses a man, a solar eclipse, and a hailstorm.
Suddenly, Kang awakens in the 21st century, inhabiting the body of Shin Seo-ri, a stand-in for actress Ji-hyo during a historical drama shoot. She fights off the actors, leaving everyone bewildered.
Meanwhile, Cha Se-gye, a chaebol heir, is embroiled in a power abuse scandal. A viral video shows him breaking things and shouting at employees at his cousin Mun-do's construction company. When questioned, Mun-do smoothly redirects the focus onto himself while remaining respectful to his cousin. We also learn that Se-gye is often insulted for having an actress as his mother.
Currently, Se-gye is running the start-up Biojei, focusing solely on money-making ventures. He takes revenge on scammers and his blind dates without hesitation. His secretary, Mr. Son, is his confidante and moodmaker.
Impressed by the deepfake technology used in the viral video, Se-gye orders Son to acquire the deepfake company. We see that he is also affected by the hate he receives. His grandfather, Dal-su, worries for him, but Se-gye doesn't think he's sincere.
Back in the present, Kang is in shock as she exits Gyeongbokgung, the preserved Joseon palace. The modern Seoul skyline with towering skyscrapers, LED screens, and vehicles stuns her.
She's in a daze when she falls in front of Se-gye's car. Se-gye assumes she's trying to blackmail him with a fake accident. Offended, she starts slapping and hitting him with a leaf. He tries to hit back with flowers but stops when people start filming them. He gives her his card for her to get checked out and leaves.
As he rants to Son, a protester blames him for abusing subcontractors. Son reveals that the logistics agency employing the subcontractors had abused their power but it has been resolved.
Elsewhere, Kang joins a field trip to a Joseon museum. One of the exhibits is her plum blossoms painting, but it is credited to Queen Mun On-jeong. The tour guide and a research student both go on about Kang the Villainess and her evil history.
Heartbroken, Kang runs into a fortune-teller who looks like Shaman Hwang. The fortune-teller believes her story and they figure that Kang is time-wandering.
Se-gye attends a meeting with a foreign investor for his new launch, Dynaestie. She flirts with him by mentioning how people slap a chaebol as a way to woo them. Son reminds him of Kang’s slap but Se-gye finds it ridiculous.
A quick summer shower passes, and Kang finally realizes she is alive. She decides to use the rich Se-gye as her shield and sword.
The protestor sneaks into the Biojei building at the same time as Kang's arrival. Dynaestie embodies Korean culture, and there is an open casting call. All the models are dressed as Joseon royalty, and Kang is accidentally ushered in with them.
Ji-hyo arrives too and once again tries to mock Kang, but the latter easily matches her energy. During the audition, Ji-hyo turns meek to appeal to the male directors, but Kang remains her usual fiery self.
Se-gye sits in on the auditions and is shocked to see Kang. He tries to make a deal with her, but she simply leaves. Undeterred, she follows him to his car.
Right then, a solar eclipse occurs, and Kang senses an ominous presence. She believes there is an assassin and drags Kang away. A dummy wearing the protestor uniform is dropped from the building, right on top of Se-gye's car. He shields her from the debris.
