Brad Pitt in World War Z.

World War Z is two-thirds of a decent movie. The movie has one helluva start, and an even better middle.

Then, in its final act, it totally craps out.

Too bad. I was looking to Brad Pitt’s zombie movie as possible relief from the mediocre, big-budget blockbusters we’ve gotten this summer (with the blessed exceptions of Iron Man 3 and Star Trek Into Darkness). Unfortunately, the much-troubled production shows every one of its scars, especially in its positively ridiculous finale.

Those who frequent movie websites know that director Marc Forster oversaw the tumultuous production, which included big delays and crucial reshoots. Lost and Prometheus scribe Damon Lindelof was brought in to write an all-new ending. Unfortunately, that ending strains so hard to be clever that you can see the throbbing veins popping out of its head. This movie called for a finale that kicked mortal ass; instead, Lindelof and Forster deliver a few minutes of Brad Pitt hanging out in a refrigerator while a lone zombie chatters its teeth.

This is one of the least-scary zombie films you are likely to see. Heck, I got better creeps out of this year’s zom-rom-com Warm Bodies. At PG-13, World War Z can’t show much blood, so scenes (including one in which somebody’s hand gets chopped off) wind up rather tame. The sense of dread ratchets down, and the film relies on pure thrills and action.

The film does deliver on the action in a killer opening sequence that sees Pitt’s Gerry Lane and his family stranded in a Philadelphia traffic jam as the zombie apocalypse launches into full swing. The revved-up zombies of this film apparently have some sort of rabies, and they aren’t interested in dinner. They just want to bite and move on to spread their contagion as quickly as possible.

Pitt’s Lane, a retired United Nations employee, finds himself in a race to find a solution for the zombie plague in order to protect his family. Pitt, a renowned family man in real life, is good for the part.

The sight of zombies forming an anthill to scale a wall in Israel, and a nerve-rattling sequence above a jumbo jet, qualify as two of the coolest sequences to show up in a movie this year. Zombies clinging to a flying helicopter and bringing it down also provide a memorable sequence. It’s a shame these scenes show up in a movie that runs out of gas.

As for reliance on the original novel by Max Brooks (son of Mel), this is pretty much an adaptation in title only. Fans of the novel and fans of bloody zombies are both going to be let down.

I won’t give away the major details of Lindelof’s screenplay tinkering. I will say that when you try to apply any kind of logic to his solutions, it doesn’t work. It seems as if they had to come up with something less expensive than a huge set piece at the end, due to the skyrocketing budget—and, as a result, the movie is a disappointment.

World War Z is playing at theaters across the valley.