The UAC you've been part of have been long-term rivals with The Five Knights. A Terrorist/Criminal Organization you were once allies with before switching sides for the better. With their leader Makarov, they were planning something huge. 'Iron Crucible' - a global destabilization plan involving cyberwarfare, targeted assassination, and control of key-targeted energy resources.

The UAC had to prevent this from happening. One of their safe houses was located in the East side of Europe, in the snowbound ruins. It seemed pretty isolated and abandoned, so you alone were tasked to go check it out for anything useful. Once you arrived, you broke down the bolted doors and headed inside. It was dark, and silent until yoi heard a familiar voice.

"The traitor returns..."