World War Z Brad Pitt, Mireille Enos, Fana Mokoena. Directed by Marc Forster. Rated PG-13. Opens Friday.

It’s not easy to do something new with zombie movies anymore, so perhaps the biggest disappointment about World War Z is the way it takes the concept of Max Brooks’ popular source novel and throws it completely out the window. Brooks’ novel is set up as an oral history, with a range of characters recounting their experiences during a 10-year war between humanity and zombies. The movie stars Brad Pitt as a character who doesn’t exist in the novel, and it takes place at the height of the zombie outbreak, trading somber reflection and political commentary for flashy action sequences and large explosions.

Pitt’s Gerry Lane is a retired United Nations investigator called back to service when a mysterious plague starts doing that thing that mysterious plagues always do in zombie movies. Concerned for his wife (The Killing’s Mireille Enos in a thankless role) and two daughters, Gerry agrees to help investigate the cause of the outbreak in exchange for a guarantee of their safety. In keeping with the title, the movie takes him from Philadelphia to Newark to South Korea to Jerusalem to Wales, and its greatest strength is in conveying the worldwide scope of the zombie infestation.

That’s partially because no one has ever spent this much money on a zombie movie before, and, at least in the first two-thirds, director Marc Forster (Quantum of Solace) shoots and paces it like the massive summer blockbuster that it is. There are some impressive action sequences, including one set in a walled-off city where zombies are literally piling up to climb over the barriers, and another set midflight in an airplane that is suddenly zombie-infested. As thrilling as they can be, they’re also a little empty, since other than Gerry, none of the characters in the movie has any depth (many don’t even have names), and Gerry himself is pretty thinly drawn, defined primarily by his pure love for his family.

Then in the final third, the seams really begin to show, as the movie slows down to become a more traditional zombie story. There were extensive rewrites and reshoots (four screenwriters are ultimately credited), and the climax often feels like it was imported from a different, less bold movie. As Gerry gets closer to answers about the nature of the undead, he ends up going through the zombie-movie motions, and World War Z loses whatever unique qualities it had. That those unique qualities are entirely separate from its source material is not necessarily a bad thing, but when the end result is so inconsistent, it’s hard not to wonder how things would have turned out if the filmmakers had trusted their initial inspiration.