In the fifth year of his marriage, Samuel Lynch falls head over heels for a college student. That young woman, Natalie Sullivan, comes from a poor family, yet she is aloof and prideful. She rejects the black card Samuel offers her and says, "I don't want to be anyone's sugar baby."Because of her words, Samuel's obsessed with her.He has been trying so hard to win the heart of the college student that everyone knows about it. However, he's forgotten about his wife, Catherine Jackman. Back then, he wrote 99 love letters just to win her heart and marry her. Catherine hasn't caused a scene. She merely burns a letter each time he hurts her because of Natalie. The day when she burns all 99 love letters to ashes will be the day she leaves him.The first letter was burned on their wedding anniversary. He didn't show up but went to the smoothie store where Natalie worked and sat there for a whole day, only to wait until she finished work.The 36th letter was burned after he abandoned Catherine on the highway in the storm when she was running a high fever. Just because he was rushing to be with Natalie, who was afraid of thunder. The 72nd love letter was burned when he replaced their wedding photo in the living room with a doodle drawn by Natalie just to make her happy.