Basically, the story line is that of the FBI Hostage Rescue Team.  The following was taken from the http://sumofallfears.ubi.com/ website:

On New Year's Eve, a band of renegade militia members from the hills of West Virginia seize a television station and broadcast their demands on the air. The F.B.I's Hostage Rescue Team is called in to deal with the situation. But this is only the first in a series of events that will take the team members from the mountains of West Virginia to the diamond mines of South Africa, and from the deserts of the Middle East to the corridors of wealth and power in Austria. At stake is the future of the free world, as terrorists unleash the most destructive weapon known to mankind.

Lead a team of domestic counter-terrorism experts through 11 dangerous missions that span the globe. Formed from the elite of the F.B.I.'s HRT, you have been tapped to perform covert ops in the most dangerous circumstances imaginable. As the commander of this team, you'll lead your unit into the heat of battle, performing missions ranging from eliminating terrorists to planting bugs, and destroying illegal weapons caches to rescuing hostages. You'll give them orders in the field and take the point in leading them into combat.