What happens when you die and come back being someone else?

24-year-old workaholic Seokjin tragically died in a car accident and woke up in 21-year-old student Jeongguk's body, still remembering his past life. Confused, he decided to see his lover and fiancee, Subin and finds out her life went downhill when he died 3 years ago.
How can he help her in a stranger's body? Will she believe him? And how can he let the love of his life move on knowing he can't?