Last year, radio host Peter Sagal was in Lexington, Ky., filming an upcoming PBS documentary on the U.S. Constitution when a man approached his film crew with a lead pipe.

"He looked like the Jesse Pinkman character in 'Breaking Bad,'" Sagal—host of NPR's "Wait Wait ... Don’t Tell Me!"—told Yahoo News recently.

Sagal was in Lexington to interview Tanya Fogle, a former University of Kentucky basketball star and ex-con whose crack cocaine addiction led to a 10-year prison sentence—and to her becoming a fervent activist for restoring voting rights to felons.

When the pipe-wielding man began to protest their right to film there, Fogle, Sagal said, "got in his face," and he backed off.

It was one of a few dicey moments during the yearlong shoot for the four-part documentary series "Constitution USA," which premieres Tuesday at 9 p.m. on PBS.

The show explores what the Constitution means in the 21st century, with Sagal crisscrossing the country on a red, white and blue Harley-Davidson motorcycle in search of ordinary Americans who "are struggling with issues of affirmative action, same-sex marriage, voting rights, the role of government and equal protection."

What Sagal discovered, he said, is that the Constitution is "not what most people believe it to be, a guarantee of our freedoms. It's our right to them."

Freedom, Sagal points out, is not included among the Constitution's 4,418 words.

“Many of us—and I include myself here—don’t really understand the document," he continued. "But that has never stopped us from arguing about what it means.”

The host pointed to an interview he did with Al Snyder, the Marine father whose son's funeral was picketed by the Westboro Baptist Church. Members of the church believe U.S. military deaths are God's punishment for the country's tolerance of homosexuality.

"My son died for the Constitution," Snyder told Sagal. "But I can't believe our forefathers meant for something like this to be free speech."

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Snyder sued the church, and the case went to the Supreme Court, which ruled in favor of Westboro's right to protest.

"There's a right to free speech, even for these hateful morons," Sagal said.

The documentary, timed to coincide with 225th anniversary of the Constitution's ratification, was shot almost entirely before the gun debate sparked by the Newtown, Conn., school shootings began. As a result, the Second Amendment does not figure prominently in the show.

"Going into this, we felt abortion and the Second Amendment had been talked to death by the usual talking heads and that nothing more could be said," Sagal said. "And until Newtown, we were right."

But despite what gun control advocates and Second Amendment rights activists believe, Sagal said, the right to bear arms "is not a constitutional issue—it's a legislative issue."

There is a wide variety of gun laws already on the books, Sagal noted, and a legislative process that exists to change them.

"So far," he said, "this is what we want."