While investigating a terroristic threat that goes viral online, Korean authorities discover that a suspect has recently boarded an international flight bound for the United States. When a healthy passenger on the same flight suddenly dies a gruesome death of unknown cause, panic erupts both in-flight and on the ground. With steadily decreasing fuel and international refusals to offer aid, the captain and crew will be forced to take unprecedented emergency measures in an attempt to save the lives of their passengers.
Dong-hyun is a high school student. One day, he falls from the rooftop and bumps into Pan-soo who is a passerby. Pan-soo is a member of a criminal organization. When the two men wake up in the hospital, they discover that they have switched bodies.
One day, 25 years ago, missing Chun-bae's letter is delivered to his cousin, Jjang-goo. The letter contains Chun-bae's lover, Bok-soon, and other people's letters. Jjang-goo finds Bok-soon to deliver the letter and meets class president Jang together. The three people recall their memories from 25 years ago, and eventually faced the person who sent the letter. Through that, the tangles begin to unravel.
