Spring Waltz is the fourth and final installment of Yoon Seok-Ho's seasonal drama series, Endless Love directed by Yoon Seok-Ho, following the preceding Autumn in My Heart, Winter Sonata and Summer Scent.
The story begins, in Austria, at midnight. Park Eun-Young is making a wish among a crowd celebrating the New Year.
The scene changes to a young Song Lee-Na watching her childhood friend Yoon Jae-Ha playing in what seems to be a piano competition. Switching back to present day, Song Lee-Na smiles remembering her wonderful memories and promises made. She begins a conversation with the passenger next to her, Park Eun-Young. She explains that she is on her way to Austria to meet her childhood love, Jae-Ha, after a 15 year separation. Lee-Na continues to explain that her first love has become an accomplished pianist as Eun-Young explains her excitement about her first trip outside of Korea.
Once in Austria Eun-Young bumps into Phillip (Jae-ha’s manager and best friend), who mistakes her as Lee-Na whom he planned to pick up at the airport. After the situation clears itself, Eun-Young, Lee-Na, and Phillip go their separate ways. Phillip takes Lee-Na to see Jae-Ha. Once at their destination, Jae-Ha doesn't recognize Lee-Na and mistakes her for a reporter. Lee-Na, upset over Jae-Ha's cold reception, begins to talk back at him. Jae-Ha later hears her name said by Phillip, which he repeats and seems to spark his memory.
While taking Jae-Ha and Lee-Na out, Phillip sees Eun-Young and decides to break company. Phillip introduces himself to Eun-Young and offers to help her get around Austria. After learning that she will be in Salzburg the next day, he offers her two concert tickets.
Later that night, Jae-Ha is seen looking through photos of himself and Lee-Na as children. Putting the photos away, he later pulls out a handmade rainbow shell.
The next day, Eun-Young meets Jae-Ha on the train ride to Salzberg. Jae-Ha displays a cold demeanor towards Eun-Young, the two are immediately attracted to each other – especially Jae-Ha, because of the way she reminds him of his long-lost childhood love, Seo Eun-Young. Eun-Young offers one of the concert tickets to Jae-Ha and introduces herself as Alice. He states that his name is Chris (Chris Y. is his stage name). They become separated once in Salzburg. Eun-Young arrives for the concert and meets Phillip, but is eager to see Chris again.
As the concert begins, Eun-Young recognizes Chris Y. as the Chris on the train. As the concert concludes, Eun-Young begins to leave the hall, but a melody Clementine stops her in her tracks as she begins to recall her childhood past.
(15 years ago) Lee Soo-Ho and Lee Jeong-Tae (Soo-Ho's father) arrive at his father's hometown, a beautiful island outside of Seoul. They meet Seo Eun-Young and her mother. Jeong-Tae leaves Soo-Ho with the mother and daughter promising to return. Soo-Ho, full of hurt and anger treats Eun-Young badly. Soo-Ho tries numerous times to escape the island. At one point Eun-Young gets caught in his plan and is taken to the hospital. Soo-Ho learns that Eun-Young is sick and is in need of a special surgery.
Soo-Ho and Eun-Young become closer; Eun-Young presents a rainbow shell she made as a gift. Jeong-Tae returns to the island and promises Soo-Ho that he will finally settle down on the island with him. Soo-Ho continues to fear that his father will leave again but hopeful of his father's promise. Later, Soo-Ho informs his father of Eun-Young's sickness and that her mother must be saving for it. Hearing about money, Jeong-Tae breaks into the family's room.
Eun-Young's mother sees Jeong-Tae running towards the ferry. Upon returning to her home, she learns that the money she was saving for Eun-Young's surgery was stolen. In disbelief and despair, she asks Soo-Ho where his father went. Eun-Young's mother leaves the island for Seoul in hopes of tracking down Jeong-Tae. While searching, she is hit by a car and is instantly killed.
Soo-Ho hurt, angry, and full of guilt tries to leave the island again by stealing a boat. Eun-Young tries to stop him but ends up going with him. The next morning, the children are swept to a neighboring island where an elderly couple take them in. Soo-Ho tries to leave again without Eun-Young but to no avail. The two make it to Seoul and begin their search for both parents.
While searching, Eun-Young becomes ill and Soo-Ho rushes her to the hospital. At the hospital, Soo-Ho mets a woman, Hyeon Ji-Sook, who mistakens him for her son. Desperate for money, he tries to steal a woman's purse. He is stopped from going to jail by Hyeon Ji-Sook's husband, Yoon Myeong-Hoon.
Yoon Myeong-Hoon asks Soo-Ho if he would consider pretending to be his son, Jae-Ha, until his wife recovers. Soo-Ho learns that the couples son, Yoon Jae-Ha, was previously killed in a rafting accident. Soo-Ho refuses but reconsiders when he learns that Yoon Myeong-Hoon has the money to save Eun-Young's life.
Eun-Young awakes to find Soo-Ho at her hospital bedside. Fearing that Soo-Ho will leave, she asks that he promise to stay by her. Soo-Ho promises but is unable to keep it. The day of the surgery he leaves with the couple. Remembering his promise to Eun-Young he runs back to the hospital but finds her room empty. Lee Soo-Ho leaves South Korea as Yoon Jae-Ha.
Jae-Ha is later told that Eun-Young died during her surgery. After the surgery, Seo Eun-Young recovers and is adopted by her aunt, Jo Yang-Soon, changing her name to Park Eun-Young.
(Present day) After the concert, Phillip convinces Eun-Young to company him to the concert's after party. At the party, Jae-Ha meets Alice (Eun-Young) again, but learns that Phillip is romantically interested in her. Aware of Phillip's past with women, Jae-Ha believes that Alice (Eun-Young) was toying with on the train. The next day, Eun-Young leaves for the Crystal Palace. She doesn't see Phillip but leaves a "Thank You" note and gift, an ornamented seashell. While Phillip and Lee-Na observe the detail of the shell; Phillip mentions Eun-Young's name causing Jae-Ha to run after her.
Jae-Ha arrives at the Crystal Palace but misses Eun-Young. Later Jae-Ha agrees to produce an album for Lee-Na's music company in Korea and returns after 15 years.
Both Jae-Ha and Phillip are looking for Eun-Young. Jae-Ha is the first to spot Eun-Young and traces her to a kimbap restaurant. While in Jo Yang-Soon's kimbap restaurant, he comes to the conclusion that this Eun-Young is not Seo Eun-Young because she refers to the owner and a worker as her mother and brother. Jae-Ha leaves the restaurant angry and disappointed. Eun-Young notices that the customer left his scarf and didn't pay for his meal. She goes after him and notices that it is Yoon Jae-Ha.
Soon after, Phillip locates Eun-Young. Eun-Young is surprised and then recalls having seen Jae-Ha. A few days later, Eun-Young is reunited with Phillip, Jae-Ha, and Lee-Na while making a lunch delivery.
Phillip, interested in Eun-Young, begins to follow her in hopes of starting a relationship. Eun-Young struggles with providing for her family and tries to find a steady job. Phillip, seeing Eun-Young hardships arranges a job for her with Lee-Na's music company as a chauffeur for Jae-Ha.
As time passes Eun-Young and Jae-Ha become closer and begin to develop feelings for each other. Lee-Na begins to sense something between the two but tries to ignore her suspicions. Phillip, unaware of Eun-Young and Jae-Ha's feelings, confesses to Jae-Ha that he is falling in love with Eun-Young. Listening to his best friend's excitement Jae-Ha is conflicted and begins to push Eun-Young away. Eun-Young hurt by Jae-Ha's behavior confronts him. Unable to contain his feelings and seeing Eun-Young hurting, Jae-Ha kisses her unaware that Philip sees them from away.
Having confessed their love to each other, Eun-Young and Jae-Ha continue their relationship in secret.
One day, Eun-Young goes to the island where she and Soo-Ho grew up together. Soo-Ho supposed she is Seo Eun-Young. But Mr. and Mrs. Yoon have told him that Seo Eun-Young was dead. Soo-Ho tries to find the fact himself. When he meest Eun-Young again in the island, he finally realizes that this Park Eun-Young is Seo Eun-Young.
Soo-Ho is also confused whether he must become Soo-Ho or Jae-Ha. When he talks this problem to the graveyard of Eun-Young's mother, Lee-Na hears it and she realizes that the man in front of her is not her childhood sweetheart Jae-Ha but Soo-Ho.
As Soo-Ho, Eun-Young, Lee-Na, and Phillip return to Seoul, Soo-Ho meets his real father, who left him when he was young. Soo-Ho's father is a drunkard. Soo-Ho is bitter toward him but he admits that he is Soo-Ho. His words are heard by Eun-Young and Phillip. So stunned, Eun-Young realizes Jae-Ha is Soo-Ho, her childhood love.
Eun-Young is angry because she thinks young Soo-Ho left her in the hospital on purpose. In reality, young Soo-Ho left to be adopted by Mr. and Mrs. Yoon who paid Eun-Young's hospital bill, but Eun-Young does not know this fact.
When she finally knows the truth, she wants to meet Soo-Ho. However, Soo-Ho is missing. But Eun-Young knows where the place he has gone. The island! So she goes to the island and meet Soo-Ho there. There is a happy reunion.
Soo-Ho cannot live easy life anymore as media discovers his real identity. Soo-Ho must choose whether he wants to be Soo-Ho or be Yoon Jae-Ha. In the press conference, Soo-Ho's real father comes to save Soo-Ho's career. He lies that Soo-Ho is not Soo-Ho but Jae-Ha. Finally Soo-Ho declares he is Jae-Ha.
His drunkard father is killed by a traffic accident causing Soo-Ho to become depressed. Soo-Ho makes a concert in Korea but he accidentally hurts his own hand by searching for his shell given by young Eun-Young in the dustbin. There is a broken glass that pierces into his hand. But he insists to play the piano.
Soo-Ho's hand's nerves becomes limped and his piano career is ended. He returns to Austria. He does not let Eun-Young know about his injury.
Nonetheless, Lee-Na comes back to Korea and asks Eun-Young to marry Soo-Ho. Lee-Na also tells Eun-Young that Soo-Ho cannot be a professional pianist anymore due to his paralyzed hand nerve. He has lost his sense of touch.
Thus, Eun-Young flies to Austria and meets Soo-Ho coincidentally on a bridge. Then they are married. Soo-Ho, who does not build a house, buys a house on the island where he and Eun-young grow together. It is the same house they lived for Soo-Ho's recording.
In the end of the movie, it is shown that Soo-Ho plays a small piano in a local school. Finally, he and Eun-Young dance in their house balcony at sunset and kiss.