"This is ridiculous."
If Helen Mirren says it, it must be true. National Treasure 2: Book of Secrets is indeed ridiculous. Even more so than its predecessor.
Ben Gates (Nicholas Cage, once again in a role that requires him to basically act like Nicholas Cage) is hot on the trail of another historical treasure, this time to clear the name of his great-grandfather, who's been implicated in the infamous assassination of President Lincoln. Don't try to figure out how those two things are connected, as the plot is just an excuse to keep the action speeding from one location to the next.
Book of Secrets follows the formula of the original movie extremely closely. There are once again hidden messages to be found on historical documents, centuries-old riddles that Ben puzzles out the answers to incredibly easily, high-security historical landmarks to waltz in and out of, and a villian that accompanies the protagonists underground for a finale based around a large action set-piece.
Riley and Abigail (Justin Bartha and Diane Kruger, both essentially just collecting a paycheck) are given nominal reasons to go treasure hunting with Ben again, and this time around he's also joined by his mother (Helen Mirren), as well as his father (Jon Voight). Mirren, Ed Harris as the antagonist, and the locations are the only real differences between the second movie and the first. Ben even finds himself once again being pursued by FBI Agent Sadusky (Harvey Keitel).
If the idea of a near-exact copy of the first movie turns you off, then I'd advise you to stay away. If you're up for more and can overlook the barely there plot and mammoth liberties taken with historical fact, then I say go for it. As for me, I don't mind switching my brain off again and watching Cage ponder ridiculous historical "facts" and Diane Kruger be hot for another two hours. National Treasure 2 is as entertaining as the first movie was, for better or for worse.