All Ghillied Up is the thirteenth Call of Duty 4: Modern Warfare single player mission. You can complete it without even alerting the enemy once of your presence (therefore completing the achievement 'Ghillies In The Mist'). The mission is a flashback from Captain Price's old days a a lieutenant. He and Captain MacMillan are in their ghillie suits and stationed in the heavily radiated area of Pripyat, following a massive nuclear reactor accident. It is the first time since WWII that the British government has authorised an assassination order. The player takes control of Price as he and MacMillan avoid the patrols to get to a vantage point on top of a hotel to get a shot at their target. They face guards and dogs on the way, but it is possible to get past them without being detected.
Just follow MacMillan and you'll be fine on this one. It's the next level you've got to worry about.