Wan's parents got into a car accident, which caused her mother to die. Her father was still alive so he wrote a letter to his friend Ae. After receiving the letter, Ae rushed to visit him. He also met Wan and still remembered her as a little girl. Wan's father asked her to marry Ae because he was a good guy and will not hurt her. She agreed without knowing his age until she saw him. As Ae and Wan's father were talking, he asked Ae to marry Wan. At first he didn't want to because she was his niece's age, but he eventually agreed and made a promise to her father.
After her father's death, Wan and Ae married quietly. She moved into his house but they slept in separate rooms. Soon after everyone finds out about their marriage, Ae decided that he was willing to let her go. At this point, Wan realized that she loved him, therefore she didn't want to leave. But Ae was unconfortable with their age gap and kept pushing her away, especially after he meets Thitaya. He decided that Thitaya was perfect for him because they were around the same age, not realizing his true feelings for Wan.
(Source: MyDramaList)