What were our Founders thinking when they gathered in Philadelphia in 1787 for the Constitutional Convention?

According to Peter Sagal, who hosts PBS's consistently lively four-part series, Constitution USA (premiering at 9 p.m. Tuesday on WHYY TV12), the distinguished delegates had both short- and long-term goals.

"The Founders came to Philadelphia to fix the Articles of Confederation," Sagal says in the first segment, "A More Perfect Union." "Also, to make sure that 200 years later, this city would enjoy a booming constitutionally themed tourist trade."

Using a panoply of interviews and some Monty Pythonlike animation, Sagal, best known as the host of NPR's quiz show, Wait Wait . . . Don't Tell Me, explores the essential role the Constitution has served in our nation, as well as the heated debates it continues to foster.

He travels from the Cradle of Liberty to Northern California, illuminating the ways the four-page document ("a masterpiece of brevity") continues to shape our lives.

A good deal of the time, Sagal rumbles around on a

Stars-and-Stripes emblazoned Harley Davidson, like some latter-day Easy Rider.

On the phone, Sagal admits that the flashy bike was more of a prop, but one he enjoyed a great deal. "There isn't a lot that can make you unhappy," he says, "if you have a new Harley on a two-lane highway in Montana."

Hard to argue with that. The Constitution, on the other hand, has started more fights than hockey.

"It's very hard for people to accept that the Constitution might actually protect something that they don't like," says Catherine J. Lanctot, professor of law at Villanova University, via e-mail. "That's not limited to lay people, either; sometimes Supreme Court justices have this issue, too."

Sagal agrees. "People are mostly enthusiastic about the Constitution when they believe, rightly or wrongly, that it supports their point of view," he says. "But the Constitution does not settle arguments, for the most part. It gives us a mechanism by which we can have an argument without killing each other."

From slavery to gun control, our democratic process has been as contentious as a divorce court.

"Almost every word or phrase in the Constitution has sparked arguments over the years," says Lanctot. "Phrases like 'due process of law' or 'equal protection of the law' or 'freedom of the press' or 'free exercise' of religion remain controversial and they always will be."

One thing that emerges in the series is that the Constitution is very likely not what you think it is.

"People are very passionate about it, even if they don't know very much about it," says Richard Beeman, a professor emeritus of history at the University of Pennsylvania and author of Plain Honest Men: The Making of the American Constitution. Beeman is one of the primary voices in the PBS special.

At one point, Sagal walked a patriotic parade route asking people about the quintessentially American credo.

"Over and over people said, 'I love the Constitution. It's about life, liberty and the pursuit of happiness' - which is not in the Constitution. It's in the Declaration of Independence."

"Most people conflate it with the Bill of Rights," Sagal continues. " 'The Constitution exists to guarantee me my freedoms.' There's nothing about the Constitution that guarantees our rights. The word 'freedom' does not appear anywhere in the document.

"What it's about is the structure for self-governing, a blueprint for a type of state through which we govern ourselves."

That doesn't mean James Madison and Ben Franklin would approve of the system we have built on their precepts.

"There's a chapter in my new book called 'The Founding Fathers Are Spinning in Their Graves,' " says Beeman.

"I truly do believe that even though they themselves were wealthy and privileged men, they would be appalled by the impact and effect of money, not only on elections but on governance. That does seem to be a fundamental violation of the central principles with which they were operating."

Couldn't we pass an amendment to light a fire under our do-nothing Congress?

"It's a difficult process," says Sagal. "The Founders made it so you need near consensus. They wanted to protect minorities."

Constitution USA is so adept at making both historical and current events seem vital and provocative that it's got Beeman, who is on the board of trustees at the Constitution Center, determined to improve the visitor experience.

(On Monday, the board appointed Jeffrey Rosen as the Center's new president and chief executive officer.)

"There's so much in that Exhibition Hall that sometimes visitors' eyes glaze over," says Beeman. "We need to involve people in interesting stories that engage the intellect and the emotions. Peter managed to do exactly that."

And he got a pretty colorful chopper out of the deal. But just as importantly, after years of rather anonymous radio work, he's ready for his star-making moment.

"I've now achieved the pinnacle of American fame," he crows. "A PBS documentary. Look what it did for David McCullough. He can't leave his house."