No anonymous dark suits wearing sunglasses and earpieces stood in the shadows scanning the crowd at National Geographic Society headquarters last night. At least none you'd notice.

But although the usual "protectees" -- the president, first family, foreign heads of state -- were not present, the Secret Service was out in full force. Even when not on duty, agents never let down their guard.

"You're always looking for anomalies in any situation," said Secret Service Director W. Ralph Basham at the reception preceding the premiere of "Inside the U.S. Secret Service." The 90-minute documentary is scheduled to air Sunday at 8 p.m. on the National Geographic Channel.

Hosted by National Geographic and the National Law Enforcement Officers Memorial Fund, the screening drew about 400 guests, most of them wanting to peek inside the agency's clandestine operations without seeming overly eager. No sense attracting attention from agents trained to notice such things. Good idea to keep your hands out of your pockets, too. And don't make any sudden moves toward the appetizers.

"It just becomes instinctive the way you observe people," said D.C. Police Chief Charles Ramsey, among a list of guests that included Jerry Parr, the agent who shoved President Reagan from the line of fire during John Hinckley's assassination attempt; and Craig Floyd, chairman of the National Law Enforcement Officers Memorial Fund, which is building a law enforcement museum to open in Washington in 2008. Program emcee Sam Donaldson, the veteran ABC News correspondent, recounted how he was standing five feet away when Hinckley shot Reagan.

But wait just a second here. What was the Secret Service thinking? Why shine the spotlight on America's vanguard of bodyguards, which historically has preferred invisibility? After all, isn't "secret" the first word in the agency's name?

"Most people know the Secret Service for protecting the president, which is what we do. . . . Our theme was always 'We say nothing,' " said Brian Stafford, the former Secret Service director who in 2001 signed off on the idea of making the documentary. "But what I wanted people to know is how special our people are and how important our mission is."

National Geographic Channel was granted "unprecedented access" to the Secret Service's unmarked Washington headquarters as the agency planned for President Bush's May trip to speak at Louisiana State University, but that didn't mean unlimited access, said John Ford, executive vice president of programming.

"There are many, many closed doors in this show," he said earlier this week. "They were the ones who said you can go here and film that and you can't go there and film that."

In the documentary, agents actually do slam doors on the film crew as they finalize plans for the president's Baton Rouge visit. They also refuse to talk about the state-of-the-art armor built into the president's limousine -- known in the agency as "The Beast." And when agents are shown being trained in split-second driving maneuvers at 75 mph at a hidden Beltsville training center, instructions aren't included.

"The film doesn't give out any trade secrets," assured Tom Mazur, the Secret Service's assistant special agent in charge of public affairs before the event.

Director Peter Schnall said: "The toughest thing was getting the agency to trust me making a film that journeys into a world that rarely opens up. To see it from the inside, it is an army on the move, and as they move, everything moves out of the way."

Yet National Geographic got enough access to tell the dramatic and timely story. "In a time when security is so important, there is clearly an interest by the public at large," said National Geographic Channel President Laureen Ong earlier this week. "And it is time these people got their day in the sun and share with people what they go through on a day-to-day basis."

The film narrative begins in startling fashion: "Assassins have attacked one out of every four of our presidents. They have killed one out of every 10. . . . The challenging fact is that someone always wants to kill the president."

Besides the Secret Service's mission, the film looks at what goes on behind the trademark sunglasses -- inside the agents' heads. Not "agents" like Clint Eastwood and others in big-budget Hollywood flicks, but real agents like Clint Hill, who when John Kennedy was shot climbed the back of the limousine to protect Jacqueline Kennedy, and Timothy McCarthy, who instinctively stood up to shield Reagan and took a bullet for the president.

"Agents are trained for that moment all their lives," said Parr, who retired in 1985 but still thinks often about that day. He thinks the movie is "the best thing that ever has been done about the Secret Service -- without giving the ballgame away."

The National Geographic documentary shows U.S. Secret Service agents and officers getting intense firearms training at a secure location in Maryland.