The Promise

Na-yeon (Lee Yu-ri) is a bright woman. She supports her boyfriend Tae-joon (Seo Jun-young), who is from a poor background, and has even given up entering a university. But Tae-joon dumps her for his success and goes with Se-jin (Park Ha-na), who is the granddaughter of the Baekdo Company president. Do-hee (Lee Yu-ri) is Na-Yeon's twin sister, who was separated from her at birth. Hence, they have never met each other. Do-hee works as a reporter at a weekly magazine, and is the girlfriend of Hwi-kyung (Song Jong-ho), who is the successor of Baekdo Company, and Se-jin's step-uncle. After a series of events, Se-jin's family wrongfully placed Na-yeon in a mental hospital. Do-hee manages to track Na-yeon down, and they switch places. Shortly after, Do-hee dies in Na-yeon's place when the building catches fire. After her death, Na-yeon decides to take revenge. She takes on the identity of Do-hee, then marries Hwi-kyung, stepping into Se-jin's family in the process.