The fourth episode of Moon River commences with Yi Kang collapsing while urgently instructing Dal-i to safeguard him. At the palace, Courier Cha Eun-woo delivers Yi Kang's message to Eunuch Yoon Se-dol, but Se-dol is intercepted by Woo-hee's henchmen. She approaches Chief Secretary Do Seung-ji with a proposition: place Je-woon on the throne so she can sit by his side. Seung-ji relays this information to the Dowager Queen Han.
Meanwhile, Dal-i escorts a semiconscious Yi Kang to a nearby shelter. As she rests, he wakes up and witnesses the injury on her hand from pressing a hot spoon to his wound. When she awakens, he insists on visiting a local physician, who treats both of them.
Courier Cha then visits Je-woon, who is currently with recovering Shin-won. Due to Je-woon's heightened surveillance, Shin-won volunteers to search for the prince instead. However, Je-woon is soon arrested after the message Courier Cha received is discovered in his residence. Han-chul interrogates him, accusing him of attempting to assassinate Yi Kang. Woo-hee attempts to threaten her own safety to persuade her father to save Je-woon, but he orders his men to confine her to her room. Han-chul also informs King Yi Hui that while Yi Kang is alive, it was Je-woon who tried to kill him, earning him the ire of both Han-chul and the Dowager Queen who covets the power to choose the next king should Yi Hui, Je-woon, and Yi Kang be removed from the picture.
As Yi Kang and Dal-i make their way to the palace, they are beset by a bandit gang and forced into a fight. Just as the bandit leader prepares to strike, Shin-won arrives in time to save them. He informs Yi Kang of the palace's turmoil, realizing that both Han-chul and the Dowager Queen desire Je-woon's demise and might even turn on the king himself, realizing that without them, she would wield the power to choose the next monarch.
One of the bandits informs them that the road is rife with threats to the prince's safety. Dal-i then calls in a favor from a ship owner, securing passage for them to Hanyang by sea. Upon arrival, time is of the essence as King Yi Hui's meal is about to be poisoned while Je-woon is about to embark on a ship Han-chul has planned to sink. In a race against time, Yi Kang sends Shin-won to save his father and warn him about the poisoned food while he saves Je-woon by the beach.
With the return of both princes, the palace resumed its tranquil state. King Lee Hui refrained from taking action against the Dowager Queen Han, aware of the potential need for her support in confronting the Left Minister. In a subsequent moment, Yi Kang revealed to Je-woon that it was Woo-hee who had shot him, urging Je-woon to keep Dal-i with him to ensure her anonymity and prevent any palace gossip.
Upon her return, Yi Kang showered Dal-i with gifts for saving his life, but she politely declined most of them, accepting only a red silk dress that he insisted she wear. Meanwhile, Je-woon confronted Woo-hee, reminding her of his disapproval of her actions.
Yi Kang set out to meet Dal-i, promising himself it would be their last encounter. As they sat down to a meal of rice soup, he asked her to join him for a fireworks display later in the evening. When Dal-i's feet developed blisters, Yi Kang went to buy her a new pair of shoes with Shin-won, leaving Se-dol to keep an eye on her.
Later that day, Minister Lee and his men passed by. One of his men recognized Dal-i as the porter from the disastrous event at Kim Ji-pyeong's house. In a moment of disdain, Minister Lee chastised Dal-i for wearing red silks that were not in line with her station. He made inappropriate comments about her 'punishment,' but rather than submit to his touch, Dal-i defiantly removed the red silk in front of the public. She called him out on his behavior, and when he attempted to attack her, Yi Kang, warned by Se-dol, intervened.
This moment of revelation for Dal-i – realizing that Yi Kang was the crown prince – also led her to assume he was only protecting her because of her resemblance to his wife and accused him of using her. He grew angry, handed her her gifts, and was about to walk away when she threw them on the ground. It was now Dal-i's turn to storm off, but she was halted by Woo-hee, who held her sword up to Dal-i's neck. Je-woon tried to stop her, and ultimately, Yi Kang grabbed the sword by the blade and threw it away.
Later, Yi Kang pays a visit to Chief Secretary Do, now incarcerated for attempting to poison the king. As he traverses the palace grounds, Han-chul informs Woo-hee that he has submitted her marriage proposal, ensuring her ascension to Crown Princess in just three months' time. Yi Kang, in his reflections, has noticed the tender glances shared by Je-woon and Woo-hee, a subtle hint of affection that begins to blossom.
Just then, he witnesses Je-woon assisting Dal-i into her aged shoes beside a babbling stream. Approaching them, Yi Kang assures Dal-i that he never used her as a mere pawn to replace his wife's memory. She urges him to apologize, emphasizing that a royal family must cherish its people and govern with wisdom, not tyranny. This enlightened sentiment echoes a previous conversation between the crown princess and Yi Kang, reinforcing the importance of compassion in leadership.
As Dal-i begins to walk away, Yi Kang pulls her back, their connection becoming electric as he grasps her wrist. Suddenly, a brilliant burst of light illuminates the sky with hues of yellow and red, startling both parties and causing them to stumble into the river below. Dal-i dives in to save Yi Kang, who is sinking but finds himself unable to rise to the surface. She even imparts a kiss to give him air, their mingling blood in the water forming a fiery red bond between them. The episode concludes with Yi Kang and Dal-i awakening after an exchange of bodies, their newfound connection burning brightly like the embers that lit up the sky before their fall into the river.
