"7-year-old Hyun-jae, always accidentally on Mirae's left side and a bit quirky, gives Mirae a toy ring before moving away, making a sincere promise to marry her when he grows up. Thirteen years later, they reunite at the same university but in different departments. As the grownup Hyun-jae once again finds himself on Mirae's left side after a long separation, their journey to fulfill ""the promise they made in childhood"" begins with twists and turns!"