Eighteen years ago, Kim Joonmyun found out she was pregnant by her boyfriend but she was too scared to tell him. So she started avoiding him, but when Yifan confronted her, she refused to tell him the truth because she didn't want to ruin his future. Thinking that she didn't want him anymore, Yifan ended things and moved back to Canada. He left Korea without knowing that Jooonmyun was pregnant with his child.

Eighteen years later, Joonmyun is 36 and a single mom to Kim Sehun. Her relationship with her son isn't the best because of all the constant dates she goes on, in hopes of finding a father figure for Sehun. After one extremely horrendous date, she runs into Yifan and his wife, who have just moved Korea from China. Even after all those years, she still finds her heart beating rapidly whenever she's around him.

A few days later, she runs into Yifan again but this time, Sehun catches them together. Yifan comments that he didn't know she had a son and starts asking about him. Joonmyun is afraid that it will create suspicion if she just leaves abruptly so she stays quiet, like she always does. Eventually, Sehun tells Yifan that he is almost 18 and Yifan does the math, figuring out that Sehun is his son. 

Sehun gets mad because his mother lied to him when she said his father was dead and Yifan gets mad because she never told him the truth. Joonmyun tries to explain herself but ends up a sobbing mess so she drives back to her old house, which is where she goes to calm down.

She ends up in her old room and decides to take a nap. As she pulls out blankets from her old closet, an old photo book falls out. It's the photo book she made when she was 17 and happily in love with Wu Yifan. She goes through the old pictures and memories start to recollect in her brain, reminding her of how happy she was back then. Joonmyun then breaks down because she realizes stills loves Yifan, even after all those years. She falls asleep in her old bed, crying and regretting what had happened eighteen years ago.

When she wakes up, she finds herself transformed back into her 17 year old self. She's been given the chance to fix the past but will she able to do it?