Sister-in-law is nineteen years old.

Young Hye-young had a happy childhood. Her father, Jung Joon-seok, was the son of a big business conglomerate boss. He married a female worker, Park Yeong-zi, against his father's wishes and they had a daughter, Hye-young. As a result, he almost severed ties with his family. Living with them were also widows Song and Kim, as well as Song's daughter, Soo-jee. Joon-seok loved Hye-young and Soo-jee dearly, often taking them out to play. On his way home to visit his mother, Joon-seok got into a car accident and lost his memory. Joon-seok's father lied to Yeong-zi, telling her that Joon-seok decided to return home to inherit the family business, and gave them a large sum of money. From then on, Yeong-zi and her daughter lost contact with Joon-seok. Unable to bear the blow, Yeong-zi fell ill. The two women living with them, Song and Kim, in order to seize her property, threw the four-year-old Hye-young onto a departing train.

More than ten years passed. The abandoned Hye-young was adopted by a fisherman family and renamed Han Jeong-min. Her adoptive father died in a sea accident, and her adoptive mother, unable to endure the hardships of life, took the family savings and left. Hye-young became the only family of her adoptive parents' son, Han Kang-pyo, who suffered from severe kidney disease since childhood. Hye-young had to regularly take him to the hospital for dialysis. To cover her brother's medical expenses, the nineteen-year-old Hye-young had to drop out of school and work various jobs. At the hospital where her brother was treated, Hye-young met the kind and handsome doctor, Kang Min-jae. Hye-young developed feelings for Min-jae and often followed him around, making Min-jae feel helpless. One day, Min-jae's younger brother, Seung-jae, ran away from home and accidentally knocked over Hye-young's fish stall. With no money, Seung-jae had to work for Hye-young to compensate for the loss. Initially, Hye-young's wild and aggressive nature made it almost unbearable for Seung-jae, but gradually he developed feelings for her.

Unexpectedly, Min-jae and Seung-jae's mother turned out to be none other than Kim, the woman who abandoned Hye-young. Another woman, Song, who also tried to seize Hye-young's family property, went bankrupt several times due to her bad habits, causing her daughter Soo-jee to live a troubled life. After going bankrupt, Song sought help from Kim multiple times and even threatened to expose the past incident of abandoning Hye-young. Having become a company boss, Kim had no choice but to comply. Before dying, Joon-seok's father entrusted him to find Hye-young. With no memory of the past, Joon-seok sent people to investigate and found Soo-jie, who lived with Hye-young during their childhood. In order to enjoy Joon-seok's fatherly love and for her own benefit, Soo-jie falsely claimed that her mother raised Hye-young until she accidentally drowned.

Kim forced Min-jae to get engaged, but Min-jae was waiting for his lost lover who had no contact. Helpless under his mother's pressure, Min-jae sought Hye-young's help to pretend to get engaged to placate his mother. They agreed on a contractual marriage, thus making the nineteen-year-old Hye-young the future sister-in-law of Seung-jae, leaving him conflicted in his feelings for her.