Da-eun (Ye-jin Son) lives with her single father Soon-man since childhood. Based on the film adapted from the kidnapping and murder of a child 15 years ago, she finds that the released phone recordings of the suspects are so similar to her father's voice. She tries to investigate her father's past. At this time, a man who seems to know about Soon-man's ulterior past appears, bringing darkness to this seemingly ordinary family....

Of course, Jae-in’s knowledge is not on the same level with others, and she protects herself in every way since she’s afraid of being seen through. Hye-ryung (played by Hong Su-a), a big sister in the class, also dislikes Jae-in and finds fault with her in everything. Jae-in has no other choice but to bear it. What made Jae-in happy from the setbacks is that she started to like a fellow student named No-young (played by Gong Yoo). However, Jae-in felt upset when thinking of her real identity and age. The operation finally made progress, the target started to believe Jae-in and she got important clues. However, something changes again....