The terrorists target is believed to be the summit conference bit it's really NSS. The conference is being used as a diversion.
Seung Hee makes a call from within NSS to alert Sa Woo that NSS is under attack. He then turns around and heads back to NSS. The system is down so Sa Woo wants to blow up the entrance to NSS.
Hyun Joon attempts to use the NSS break-in to find a way to break the lock on the file on the drive. He runs into Seung Hee. He has a mask on so she doesn't know it's him. A terrorist attempts to sneak up on Seung Hee but Hyun Joon shoots him. Seung Hee thinks he's trying to shoot her and opens fire.