National Geographic: Inside the U.S. Secret Service

"The first and last line of defense for the president."

Even though they are frequently seen flanking presidential convoys in their trademark dark suits and sunglasses, little is truly known about the insulated world of the United States Secret Service. National Geographic's Inside series delves deep into the inner workings of the secretive organization for an unprecedented behind-the-scenes look at the upper echelons of national security.