Remember those commercials where the yard-sale scavenger finds a signed copy of the Constitution beneath a bucolic painting? Well, "National Treasure" does that enviable discovery one better.

In this "action flick meets history lesson" hybrid, treasure-hunter Benjamin Franklin Gates (Nicolas Cage) learns that a map to the fortune to end all treasure hunts is on the back of the Declaration of Independence.

How it got there is a not-quite-settled stew prepared with dollops of conspiracy theories and dashes of fact. The tale begins with the Knights Templar, a group of warrior monks founded at the end of the First Crusade, and ends with this nation's Founding Fathers, some of whom were Freemasons - hence the coded chart on the back of one of our founding documents.

Ben's grandfather (Christopher Plummer) imprinted the saga on the young Ben's psyche. Around the nation's capital, the Gates family has a reputation for being mildly cracked. Ben's father (Jon Voight) took and takes an aggressively anti-treasure stance.

So on a more personal note, Ben's discovery proves that the previous generations of Gates men (there are no Gates women) weren't just talking crazy.

There are wrinkles aplenty to Ben's discovery. First, the legend is written in invisible ink. Second, one of the finest documents to have ever been penned is protected by an elaborate security system at the National Archives in Washington.

Diane Kruger ("Troy") plays a conservator at the Archives with the appropriately historical moniker Abigail.

Third, Ian (Sean Bean), the friend who financed much of Ben's search for the hoard, is now Ben's nemesis, and he's willing to steal - even destroy - the Declaration to get at the map.

That would be identity theft on a national scale.

Laudable, at times inventive, "National Treasure" relishes the idea that kids can be thrilled by an intellectual caper. Clues and riddles drive Ben and his sidekick tech whiz Riley (the very appealing Justin Bartha) toward their goal.

Too bad the movie doesn't always have the courage of its convictions. In the midst of celebrating brains, the filmmakers can't help exploiting the usual brawn of the action flick. At times, "National Treasure" seems more like a kiddie primer to future Jerry Bruckheimer fare than a clever journey through history. Here's when things blow up. Here's when bullets fly. Here's when the motley Euro-baddies mumble in their dunderheaded accents.

Fortunately a number of national monuments make their cameos with more eloquence.

In our post-9/11 world, the suspicious comings and goings near Independence Hall and the Lincoln Memorial may be a bit hard to swallow.

But then it's easy to see how "National Treasure" is a generous response to that tragedy.

Look kids, the film asserts, the treasure of the title is yours in these public spaces. It's embedded in the language of a piece of parchment.

"Here's to the men who did what was considered wrong, in order to do what they knew was right," says Ben, a noble sentiment expressed just before stealing away to steal the Declaration.

TITLE: "National Treasure"DIRECTOR: Jon TurteltaubCAST: Nicolas Cage, Jon Voight, Harvey Keitel, Diane Kruger, Sean Bean, Justin Bartha, Christopher PlummerRATED: PG (action violence and some scary images)GRADE: * * * (on a scale of 5)