In the 7th episode of SBS drama "Ghost" that was aired on the 20th of June featured Yoo Kang Mi (Lee Yeon Hee) working on an investigation at the high school she graduated from. 

At the high school, two students had committed suicide. Yoo Kang Mi begins to remember her experience while she attended the same school. Yoo Kang Mi was always at the top of her class when it came to grades. One day however, one of her friends Kwon Eun Sol performed better and Yoo Kang Mi placed 2nd. From her 'defeat', Yoo Kang Mi was shocked.

At the high school, there was a 'Legendary Answer Key' rumor that floated around. It was said that a ghost of a student who died from stress shows the complete answer key to the final exam to one student who is attending the school at the time every year.

When Kwon Eun Sol placed first unexpectedly, students began to suspect her of receiving all the answers from the ghost. As a result of jealousy, other students locked Kwon Eun Sol in a classroom to keep her from attending class. Yoo Kang Mi heard Kwon Eun Sol screaming out, 'I'm going to fail if I don't go to class!' but Yoo Kang Mi ignored her. After the event, Kwon Eun Sol chose to commit suicide.

Yoo Kang Mi remembers, "Our school cost $8,600 (10,000 Won) per semester, all the students and their parents sacrificed so much to attend the school. We dedicated every part of our lives to studying. Eun Sol's family didn't have that much money. They had to lend money to pay for school but because of us, she failed and chose to kill herself."

It was also revealed that Yoo Kang Mi felt responsible for Kwon Eun Sol's death and was thinking of committing suicide as well.