Before you see this movie, you should really do yourself a favor and read the book. "The Relic" is basically the dumbed-down version of "Relic," the novel by Douglas Preston and Lincoln Child, an amazing and scary read. For the general moviegoer, The Relic simplifies the story significantly. Missing are two of the most influential characters, Special Agent Pendergast and the journalist, Bill Smithback. Greg Kawakita becomes Greg Lee, and his character is so much simpler in the movie. Mrs. Rickman, Ian Cuthbert, and Dr. Wright are combined into a single character, a female Dr. Cuthbert. Whittelsey becomes Whitney (presumably for the ease of the characters' speech). Also, the fictional museum is moved from New York to Chicago (why, I cannot say).
Basically, the plot is the same: people start turning up dead, horiffically slashed and decapitated, on the eve of the opening of the museum's new Superstition Exhibit. No one believes it's a monster, but of course, it is. Duh. Chaos ensues.
Let me take a moment to talk about the monster. In the novel, the monster, Mbwun, is terrifying. When it is finally seen, it is revealed to be quite manlike... which of course makes sense, given its origins. However, in the movie, the creature (whose name is changed to Kothoga, probably to aid in pronunciation again), while wicked awesome cool, just doesn't fit with the story. It'd be better off in an action movie, not a straight horror. In fact, the scariest parts of the movie occur before the monster is even seen (which is the thing with most of these movies).
Bottom line: as a movie, "The Relic" is quite entertaining. Compared to the novel, it falls short.