Mash-ups have often been fun diversions. Think of music mash-ups such as “Happy Days are Here Again” with “Get Happy,” or any number of more modern ones (watch more than one episode of Glee and you’ll see at least one.) Film mash-ups combine genres, which is often an exercise in exhilaration or frustration. “It’s a Western—no, it’s a musical. No, it’s both!” (Paint Your Wagon) Or think of David Lynch, or anything by Tarantino. Sometimes fun, sometimes fascinating, sometimes confusing.

World War Z begins as a promising mash-up of a family drama interwoven with a zombie movie. Although it gets to the central conflict rather too quickly, it works in an oddly fascinating way, mostly because the film does two things well. First, it keeps rooted with touching scenes of family connection and concern, and doesn’t rush them. Brad Pitt plays Gerry, a UN superman called to save the world in the light of the newest zombie pandemic. But the film, while showing his mad skills, leans hard toward Gerry as loving husband and protective father, and for a while, that connection adds depth and keeps the film rooted in the real world of love, support and family. Second, the film keeps the zombies at something of a distance by not reveling in the gore of the attacks and keeping the worst of the action just out of sight. Yes, it’s about zombies, but it might be any apocalyptic nightmare scenario.

But just when you think it all might work, the film s-l-o-w-s d-o-w-n and becomes a drawn-out suspense thriller that takes the film in the direction of another mash-up, this time a medical thriller (spoiler alert: will they find the proper antidote?) combined with a long, drawn-out sequence of equal parts zombie suspense and messianic sacrifice. Then it doesn’t wrap up as much as it just “concludes” with an open-ended narrative shrug that comes somewhere between “whatever…” and a “I dunno….” By this time, the promise of the first half has long been forgotten, the pace has been recalculated one too many times, and the two strong strands of family and zombie are left dangling and unresolved.

The film’s acting is its saving grace. Brad Pitt has become a better, deeper actor since becoming a father, and the changes work well here. Though he’s still hiding his looks under scruff and long hair, he seems to have left behind the quirks of the character actor and has embraced his leading actor status. He’s strong, smart and paternal all at once, and he holds the film together. His wife is played by Mireille Enos in what I hesitantly call the Jessica Chastain role (a category that I hope doesn’t exist a year from now). Her role isn’t as flashy, but it’s solid, and contributes greatly to the family dynamic of the film.

Two spoiler alerts ahead: The film does have one genuine stop-your-heart moment amidst a number of other moments of surprise, plus there is a narrative twist toward the end when the film presents its “answer” to the crisis. It’s fresh and imaginative, but it’s nearly lost in the missteps of the second half.

Not being able to categorize a film is not a criticism. Citizen Kane can’t be categorized, either. But nearly everything in that film works. WWZ starts off strong with a fresh mash-up of family and zombies, then careens into a series of narrative bumps and unrelated set-pieces that ends up derailing the whole thing by the time it’s over. The acting is good, the production values are generally first-rate (though the zombie hordes look CG), but the final effort is most certainly less than the sum of its parts.