The most suspenseful sequence in “National Treasure: Book of Secrets” has the American treasure hunter played by Nicolas Cage masquerading as a local and haranguing a London bobby in Buckingham Palace. Your breathing becomes very rapid and your knuckles start to pale as you think: How many more lines can Cage keep it up with his idea of a Cockney dialect?

Compare that scene to the big one near the end of this very draggy sequel. Cage, Jon Voight (as his pop), Diane Kruger (as his ex), Justin Bartha (as the Jimmy Olsen of the piece), Helen Mirren!? (as mom) and Ed Harris (as a rival treasure seeker looking for the lost City of Gold) have converged inside Mt. Rushmore, which was of course a massive cover-up designed to conceal the golden temple. The set looks like a pre-Columbian water park straight out of the Wisconsin Dells, and director Jon Turteltaub keeps us in there a long, looooooonng time.

In the first “National Treasure,” which was a good-sized international success, Cage and associates chased down treasure-hunt clues encoded in the Declaration of Independence and the Liberty Bell. Here the clues lie in the Statue of Liberty (not that one, the other one) and in matching antique desks located in Buckingham Palace and the White House Oval Office. The film slogs all over the map, from Paris to London to Rapid City, S.D., in hopes of recapturing some of the first film’s box office appeal. The plot also has to do with missing pages from John Wilkes Booth’s diary, and Ben kidnapping the current U.S. president in order to get a look at the super-secret Presidential Book of Secrets.

All you want from a movie like this, really, is a little brainless fun, and it keeps holding out on you. Everyone looks fatigued. Even Cage’s toupee seems ambivalent about having signed on for a sequel. The script by The Wibberleys keeps spinning the compass, and a line spoken with weary authority by Voight in the first “National Secrets” haunts this one: “And another clue leads to another clue ...”