Their friendship dates back to elementary school, when they were both students at the same elementary school. To his surprise, Yi-seul had been in love with Baek-ho for more than two decades without his knowing. Finally, at Yi-wedding, seul's Baek-ho regrets that he didn't tell her how much he loved her before she married another guy. Yi-heart seul's is offered a second chance by a mysterious conductor. In an attempt to alter the course of their love, Baek-ho goes back in time to various moments in their lives. But it's not as simple as it appears to change the future.