In the opening episode of Moon River, a young boy dashes towards an aged figure known as Elder Wolha. The boy presents him with a book, and Wolha deftly copies two names from its pages. These names float towards a radiant tree, where they blossom into one of the many flowers that dot its branches. Wolha explains that these are fate blossoms, representing the red thread of fate that binds two individuals together. The boy inquires about a peculiar flower, different from the rest, prompting Wolha to embark on a tale of a couple connected by the thread.
We witness Crown Prince Yi Kang meeting a captivating woman and succumbing to his passions. They marry, but as Wolha narrates, greed intercedes to disrupt their happiness. The crown princess, in the twists of fate, eventually passes away.
In the present, Yi Kang indulges in a tantrum over his new royal robe's color, which doesn't match his exact preference—a system he introduced five years ago that assigns specific hues to each individual. During his meeting with the ministers, they debate the matter of remarriage for the crown prince. Yi Kang announces his decision to marry the daughter of Left Minister Kim Han-chul, who was always the prime candidate.
Meanwhile, King Lee Hui is kept isolated from the others in the palace due to Kim Han-chul's machinations. Han-chul seems to wield immense power over the king, though it remains unclear why. Lee Hui muses on whether Han-chul harbors plans to eliminate Yi Kang after he marries his daughter.
Through conversations among Yi Kang's entourage, we learn that the crown princess was falsely accused of involvement in the queen's assassination. She was given poison but, in protest, drowned herself. Her decomposed body was found much later, and Yi Kang identified her ring. This is why he now harbors an aversion to all aquatic creatures.
A flashback takes us five years back to see Yi Kang imploring his father to treat his consort's death as a murder and initiate an investigation. Lee Hui confesses that he ordered her to drown herself out of fear that Yi Kang would either flee with her or die with her.
In the present moment, Yi Kang carries the new royal robe to his wife's grave. On the brink of consuming poison, Kim Han-chul stops him and lays bare his nefarious plan—he killed the queen and framed the consort, knowing he holds too much power for Yi Kang to act against him.
In the quaint town of Uljin, Park Dal-i, a resilient woman with a salt-stained legacy, traverses the neighborhood, door to door, selling her wares. Her journey is a testament to survival, for she was found near the brink of drowning and has since lost all memories of her past. Mr. Heo, a steadfast naval commander and a frequent patron of Dal-i's, calls upon her with a request: to disguise herself as a peddler and investigate his daughter's well-being in the neighboring town of Hanyang. His daughter's cryptic letters hinted at a desperate plan, leaving Mr. Heo anxious for her safety.
With a heart full of resolve, Dal-i packs her belongings and prepares for the journey that her aunt Park Hong-nan urges her to avoid at all costs. Hong-nan's warning echoes of the dark past where Dal-i was once a slave and Hanyang's slave hunters still roam. Nevertheless, Dal-i embarks on her mission.
Meanwhile, in the palace's hallowed halls, Yi Kang's grandmother, the Queen Dowager Han, confers with a minister, their conversation fraught with intrigue. She frowns upon Yi Kang's proposed marriage with Han-chul's daughter, fearing it would grant the minister undue influence. The conversation also reveals the kingdom's complex history; before Lee Hui's reign, his brother ruled but met an untimely end alongside his second-born son on the same fateful day. Je-woon, the late king's firstborn, still lives but under the Queen Dowager Han's shadow, whose ruthless hand saw Je-woon's mother framed and executed, and Je-woon himself deposed.
Dal-i finally locates the house she seeks and manages to see Mr. Heo's daughter. The girl confides in her that she is being coerced into ending her life. Dal-i devises a plan to save her without alerting anyone else, whispering instructions in the girl's ear.
Later, amidst the bustling market, Yi Kang spots Dal-i and mistakes her for the crown princess. Dal-i, fearing she has encountered one of the dreaded slave hunters mentioned by her aunt, flees in a panic. She stumbles upon a small hut brimming with lush plants and takes refuge within its confines to change her attire. However, she is interrupted by a man perched on a tree inside. As their eyes meet, he drops his clock, which sends it crashing into Dal-i, rendering her unconscious.
As she stirred from slumber, he interjected, seeking retribution for the clock and commanded his servant to detain her. Surprisingly, this man turned out to be none other than Je-woon, Yi Kang's cousin brother. Yi Kang's visit was met with accusations of theft leveled at his sibling, and upon learning of the clock's disrepair, a physical altercation ensued. The others fled the scene, leaving it clear that this was the turning point. It seemed that Yi Kang had been masquerading as a ruffian indifferent to governance, all the while scheming a revenge against Han-chul.
Meanwhile, Dal-i sneaked into another woman's attire and attempted to make a run for it. The entire household pursued her, and she leapt from a rooftop...straight into Yi Kang's waiting embrace. Upon returning home, Dal-i's adoptive father inquired of Hong-nan as to why Dal-i must not venture to Hanyang. Hong-nan revealed that Dal-i would be doomed if recognized there.
