Born with a weak heart, Shim Hyewon is envious of her school friends being able to run freely. Suddenly, she collapses in the rain. Meanwhile, Yoo Minwoo, a college student, runs for cover in the rain and encounters So Eunhye. Unfortunately, Eunhye dies on the day of her wedding to Minwoo. Fast forward many years, and Hyewon is living as a florist. She and her friend, Oh Jangmi, visit the mountains to collect wildflowers. Hyewon gets separated from Jangmi and becomes lost in the woods.

Minwoo and Hyewon meet in the mountains. Minwoo is attracted to Hyewon who is a lot like Eunhye. Hyewon's heart flutters fast when she's with Minwoo. The two end up spending the night together in a hut, and Hyewon gets her leg injured while coming down the mountain. Worried, Jangmi calls up Hyewon's fiancé Park Jeongjae, who rushes over. At the foot of the mountain, the three of them meet.

Minwoo and Hyewon run into each other in a tea field and are surprised by the coincidence. It's almost as if Hyewon had been there before. They rush to catch a bus and barely make it to the train station on time, but the tickets are sold out. Minwoo offers his train ticket to Hyewon. Hyewon falls asleep on the train and wakes up to see Minwoo in front of her. When they arrive in Seoul, Hyewon offers Minwoo her business card, before she catches a glimpse of Jeongjae waving at her.

The four visit Minwoo's favorite field of wildflowers. Park Jeonga catches Minwoo and Hyewon together and mistakenly thinks they are kissing. Jeonga, unable to contain herself, lashes out at the two, only to find out that it was a misunderstanding. Minwoo and Hyewon are teamed up for a renovation project. Minwoo asks Hyewon to be his date for the opera. Hyewon hesitates at first but agrees to go as a friend. Jeongjae also gets tickets to the opera for Hyewon.

The four arrive at Calla Resort. Jangmi and Ji Daepoong frequently argue, but Minwoo and Hyewon always voice singular opinions. Jeongjae tells Minwoo that he should forget his former flame and pursue a new love interest. Meanwhile, Minwoo's feelings for Hyewon grow stronger. Jeonga finds the two taking photos and expresses discomfort about the fact that she had seen them nearly kissing. After the misunderstanding is cleared up, the four people enjoy a cheerful time at the lake.

Hyewon finds out how Minwoo truly feels about her. In order to push Minwoo away, she tells him that she's already part of Jeongjae's family. She also tries being cold to Minwoo and tells him that she doesn't even want to be friends. Minwoo perceives that Hyewon is angry, and tries to cheer her up, to no avail. The shop owners at the mall start to develop ill feelings towards Jeongjae. Minwoo feels sorry for the business owners, but Hyewon sides with Jeongjae on the matter.

Jeongjae visits Hyewon, who is working on the \"Proposal Room.\" He sees Hyewon warming up to Minwoo and feels left out. Jeongjae proposes to Hyewon in the room, but Hyewon turns him down. Meanwhile, Jeonga discovers a necklace in Minwoo's room, only to get upset when she finds out that Hyewon already knows about the necklace. Jeongjae discovers that his necklace is missing, and searches for it. Hyewon forgoes an appointment with Jeongjae to help find the necklace.

Hyewon passes out at her engagement ceremony and Jeongjae becomes very concerned. Minwoo finds out from Jangmi that Hyewon has had a heart transplant. Minwoo rushes to Hyewon's side at the hospital. Meanwhile, Hyewon is in a coma when she dreams about a strange man calling out for her. Minwoo looks on as Hyewon is comforted by the mysterious voice. He leaves behind a picture he drew of Hyewon. When Hyewon wakes up, she finds Jeongjae watching her anxiously.

Jeongjae discovers Minwoo and Hyewon spending together at a park and decides he must take action. Jeongjae visits Minwoo and asks him to forget about Hyewon. Minwoo is hurt but decides to stop having feelings for her. Meanwhile, Minwoo tries not to be alone with Hyewon. When Jeongjae becomes able to confirm that Minwoo is no longer pursuing Hyewon, he is relieved. Minwoo tries to be cold to Hyewon on the job. Hyewon meets Minwoo at the folk-dance event Jeonga has organized.

Jeongjae and Jeonga become worried when Hyewon doesn't return from a trip she took alone to an island. Minwoo gets concerned and rents a motorboat to go after Hyewon, only to find her acting cold toward him. Despite such a demeanor, Hyewon is actually grateful and happy to see Minwoo. Jeongjae is nervous about Minwoo and Hyewon, but he is relieved to hear from Daepoong that Minwoo has returned to the resort. Minwoo secretly finds a place for Hyewon to stay at.

When Jeongjae sees Hyewon getting off the boat, he tries to stay calm. But Jeonga gets angry and tells Hyewon she won't forgive her. Meanwhile, Daepoong updates Minwoo on what had happened and advises him not to do anything to worsen the situation. Minwoo tries to protect Hyewon, but Jeongjae gets enraged and punches him. Meanwhile, Jeonga asks Hyewon to break up with her brother. Hyewon, feeling guilty, tries to break up with Jeongjae, but he refuses to.

When Minwoo hears how Hyewon has left home alone, he takes Daepoong's car in search of her. Jeongjae, desperately waiting for Hyewon, asks Jangmi to call him if Hyewon returns home. Jeongjae returns to the resort to focus on renovation efforts, only to be hurt when he hears Jeonga mention Hyewon may be with Minwoo. Eunhye's father comes to meet Jeongjae at the resort to ask that his wife wishes to meet the recipient of Eunhye's heart, but Jeongjae turns him away.

Hyewon runs away from the summer home and hops into Minwoo's car. Jeonga cleans up Jeongjae's mess, only to discover Eunhye's dad's message with \"So Eunhye" \ written on it. When Jeongjae realizes that Hyewon's heart donor is Minwoo's first love, he rushes to the hospital to get confirmed facts. Jeongjae meets Eunhye's dad and asks that he doesn't look for Hyewon. When Minwoo doesn't take her calls, Jeonga calls Minwoo's mother and discovers that he will be going to the flower field.

Hyewon comes across Eunhye's grave. The former thanks the latter for donating her heart. Minwoo watches this from afar and gets saddened. Hyewon tells Eunhye's dad that she'll come to visit him from time to time, but Eunhye's dad insists that's not necessary. Later, Minwoo visits Eunhye's home, and Eunhye's mother asks Minwoo if he had forgotten all about her daughter. Minwoo is so sad that he spends the night awake in Eunhye's room.

Hyewon finds out from Jeonga that the heart donor was a woman. Hyewon calls Eunhye's dad and leaves a voice mail. Hyewon asks Jeongjae whether the heart donor was female and wonders why he didn't tell her before. Eunhye's dad meets with Jeongjae and tells him that since Hyewon knows the truth, he could tell her about Eunhye's heart. But Jeongjae stresses that Eunhye's dad must never tell Hyewon the truth because Minwoo and Hyewon are in love.

Hyewon finally finds out about the secret surrounding her heart donor but finds herself unable to retract her feelings for Minwoo. Eunhye's dad tells Minwoo to share his heart with Hyewon. Minwoo tells him that he'll be happy with Hyewon and do his part to live on behalf of Eunhye. Hyewon tries to avoid Minwoo but ends up meeting him in an awkward situation.

Hyewon tells Minwoo that they must break up, but Minwoo is unable to believe her decision is a genuine one. Hyewon refuses to be overshadowed by Eunhye. Jeonga is glad that Hyewon found out about her heart donor but does not alert Minwoo of the fact. Jeongjae is upset when thinking about how hurt Hyewon would've been when she found out the truth. After a medical checkup, the doctor asks Jeongjae whether Hyewon had recently experienced a shock that worsened her heart condition.

When Minwoo finds out Hyewon is already aware that Eunhye had donated her heart, he is taken aback. Minwoo rushes to talk to Hyewon, but Hyewon treats him coldly. Unwilling to take Eunhye's place, Hyewon fights back her feelings for Minwoo. Jeongjae finds out that Minwoo had come to visit Hyewon and is worried that Hyewon may waver. However, Hyewon declares to Jeongjae that the person Minwoo loves is Eunhye. Meanwhile, Minwoo comes across Hyewon's wedding invitation by chance.

Minwoo waits for Hyewon at the wedding venue, only to find out from Jangmi that Hyewon has passed out. Shocked, Jeongjae rushes to the hospital, only to find Minwoo already waiting there. Jeongjae grabs Minwoo by the collar and blames him for everything that has happened. The doctor tells Jeongjae that Hyewon is in critical condition. While everyone waits for Hyewon to gain consciousness, Jeongjae tells Minwoo to leave.