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Hajun's dad is a high end weapons researcher and designer (think Nuke's etc). He is currently on something HUGE that everyone who is everyone wants to get their hands on.
Hajun's dad has declared that he will put his inventions up for auction, but I guess Candy's bosses don't wanna pay. So they are planning to murder him and steal the stuff before it goes to auction
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Shihyeon's job here is to seduce Hajun and find his dad's location, because what dad won't stay in touch with his family even if he went off-grid from everyone else..
But as Shihyeon later finds out, the guy maybe a genius weapons maker but he was a deadbeat dad who hasn't called his son in 20-ish years.
so i have been reading for a long while, but i still havenot figured out what Shihyeon is trying to accomplish, maybe ive been blind sided with all the cute romance and sex scenes, but what does Hajun or Hajuns father do that makes the spies so interested in them?