Kim y/n , she works in Chiel Worldwide company Korea's number 1 marketing company,  a subsidiary of Samsung Group ...she always visits Busan her hometown to meet her grandma .. her parents took divorce when she was 12 y/o after her parent's divorce, her grandma is the only who took care for her , if she can consider someone as family then her grandma is the only one..
She had to move to  Seoul due to company but she still visits her grandma.

On every Saturday after finishing her work at 6 pm she goes to railway station, after 6 pm there's only  one train left for Busan which arrives at 10:30pm usually there's few people in train at this time
On first 6 months everything was going well untill she noticed a  strange person in train ... In starting 2 months she just shrugged off but on
12 Dec 2018 she didn't