In high school, the free-spirited Jung Da-ae and the diligent Lee Han-eol despised each other. Now, their older siblings—Mirim and Tae-gang—have fallen in love and decided to get married, though it does not quell the eight-year-long hatred between the two bitter enemies. Tragically, Mirim and Tae-gang die in a car accident on their wedding day, leaving their loved ones devastated. As Da-ae and Han-eol get into yet another fight during the funeral, a speeding truck hits them, and they awaken in their teenage bodies with their memories intact.

Da-ae and Han-eol still cannot see eye to eye, but there must be a reason they were sent to the past together. When they find Mirim's diary from the original timeline, detailing the beginning of her relationship with Tae-gang, everything starts to make sense. For the first time in their lives, Da-ae and Han-eol agree: to save their beloved siblings, they must prevent them from ever meeting.