Young-ho doesn't have any dreams for his life. He has been studying for three years to enter a university. He decides to send a letter to his childhood friend So-yeon. But her younger sister So-hee receives the letter instead of her sick sister. So-hee writes back to Young-ho, pretending to be So-yeon. Meanwhile, So-hee takes care of her sick sister and also runs a secondhand bookstore with her mother.
After being wrongly imprisoned for murdering her husband, a woman returns to the house where the incident occurred to solve the mystery of his death.
A hitman accidentally kills a little girl. Filled with regret, he wants to quit. But then to tie up loose ends, he is forced to go on another job, to kill the girl's mother.
Joon-seok is sent to prison for his involvement in Dong-soo's death. He meets Dong-soo's son in prison, keeps his identity a secret, and agrees to join forces when he is released. All of their plans fall apart when Sung-hoon finds out the truth.