If you had a second chance to correct the biggest mistake of your life, would you take it? Ai Li Si is a cold, 30-year-old woman who lives her dull life like a robot programmed to do the same things every day. But her boring life changes when she wakes up one day to discover that she has returned to her 17-year-old self! Back in high school on the day of an important swim meet for her best friend, Shu Lei, Ai Li Si literally runs into the moody He Hao Yi, who has just transferred to her school. Hao Yi mistakes Ai Li Si for his long-separated twin sister, and because of a grudge against her and his mom, Hao Yi decides to torment her at every turn. Meanwhile, Han Ming is a nerdy student who has long harbored a secret crush on Shu Lei. After a few mishaps, Hao Yi realizes that Ai Li Si is not his sister, but will Ai Li Si take this opportunity for her to correct her biggest mistake with her first love and change her own future destiny?