I loved the first movie, but the second one sucked and was far too formula driven. Here's a rapid synopsis:
Cage and his girlfriend break up, and he moves out of the giant mansion. His friend's sports car gets towed. So much for all the money they made. They now set out on a new adventure: to prove that Cage's character's ancestor didn't conspire to assassinate Abraham Lincoln.
The "evidence" is a fragment of a single piece of paper, listing Gates with other conspirators. Truly underwhelming evidence, but nevertheless evidence. In the course of getting to the truth, they prove the ancestor's innocence, get filthy rich again, Cage's character and girlfriend reconcile, the assistant gets a girlfriend, and even Cage's parents (played by Jon Voigt and Helen Mirren) reconcile after a bitter 30 years of being happily divorced. Once again, the man is the thoughtless pig that ruins the relationship.
See this movie only if you're a big fan of Mirren (which I am) or if you like the cool riddles they have to solve.