Robert Zemeckis has, at the very least, had a fascinating career. Starting off with writing 1978’s teen pic “I Wanna Hold Your Hand,” he became a Spielberg protege, directing a number of big blockbusters, including one of the most beloved of all time, “Back To The Future.” Then he switched into more prestige-y fare in the 1990s with the Oscar-winning “Forrest Gump” and the less adored, but less stodgy “Contact.” Then he spent the 2000s abandoning reality altogether with a series of performance-capture CGI animations that often felt more like theme park rides than actual movies.

That phase wrapped up about six years ago, and since then Zemeckis has returned to live-action with decidedly mixed success: “Flight” was an interesting idea executed in a resoundingly dumb manner, but proved a sleeper box office hit, while “The Walk” was often astounding, but sort of redundant in the face of the acclaimed documentary “Man On Wire,” and tanked incredibly hard. Could “Allied,” a sort of classical World War II spy romance with a couple of mega-hot A-listers in the lead, be his unreserved comeback? Alas, not entirely: it’s an engaging film in many respects, but one that exemplifies a lot of the problems that have trailed Zemeckis across his career.

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In 1942, Wing Commander Max Vatan (Brad Pitt), a Canadian in the SOE, the British wartime precursor to MI5, parachutes into Nazi-occupied Morocco. He sneaks into Casablanca where he meets his resistance contact Marianne Beasejour (Marion Cotillard), a woman who who’ll pretend to be his wife as part of a mission to assassinate a high-ranking Nazi official. They try to ignore their growing attraction to each other, but expecting to be killed on the morning before they execute their plan, end up sleeping together.

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But then the plan goes off without a hitch, and Max asks Marianne to return to London and marry him. A year later, they’re living happily in Hampstead with a baby daughter, with Max behind a desk at the SOE. But one day he’s called in by a mysterious superior and told something shocking: Marianne is suspected of being a German agent and a traitor. He’s instructed to leave some false information around the house, and if decoded traffic suggests that the information has been passed to the Nazis, he is to execute his wife, or he’ll also be killed for assisting her. And so begins the longest 72 hours of his life.

Brad Pitt plays Max Vatan in Allied from Paramount Pictures.

“Allied,” penned by “Eastern Promises” and “Peaky Blinders” writer Steven Knight, is in some ways a film of two halves. The first is a gripping if not wildly inventive classically-minded WWII resistance thriller in a glamorous, but darkly-tinted setting, with a flirtation that grows into something deeper. The second is a sort of psychological relationship thriller that sees Max fight to clear his wife’s name, even while the seed of doubt remains firmly planted in his mind. And they’re both often engaging, well-written and well-performed — “Allied” is rarely less than eminently watchable.

But the duality isn’t limited to the structure: “Allied,” ironically enough, given its title, is a movie that’s often at war with itself. Knight’s screenplay would appear to point towards a taut, intimate affair, one that really digs into its two central characters and gets into the white lies and deeper deceit that can take place in wartime and in marriage. But whether by the mandate of a studio wanting “trailer moments,” or a director who’s always been happy to play with shiny toys, the film is constantly undermined by the actual execution.

Marion Cotillard plays Marianne Beausejour in Allied from Paramount Pictures.

Does Zemeckis begin the movie with an egregiously fake CGI parachute jump? Yep, and it instantly undercuts the reality of everything that follows. The characters can’t just have sex, they have to sex in a car surrounded by an egregiously fake CGI sandstorm, the camera whirling around them impossibly. An egregiously fake CGI plane randomly crashes near Max and Marianne’s house, and Pitt single-handedly flies behind enemy lines in the third act in a silly detour that appears to be made up entirely of cut footage from his last WWII picture, “Fury.” It’s like trying to watch Hitchcock’s “Notorious” except that someone has malevolently spliced scenes from “Pearl Harbor” intermittently throughout.

The result is that big-budget bluster careens through the emotional premise of the movie, and it’s not helped by the casting. Cotillard is great, but the machinations of the plot keep her offscreen and/or inscrutable for much of the running time. And Pitt is a great actor, but he’s usually more effective when he gets to let his freak flag fly. When he’s supposed to be emotionally engaging, there’s often something detached about him, and that’s the case here, and its a detriment to the picture as a whole.

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Even the writing, which is mostly strong, keeps shooting itself in the foot. The idea that Pitt has a lesbian sister also in London, played by Lizzy Caplan, isn’t bad in and of itself, but the film never does anything with her, and it ends up feeling like incongruous tokenism as a result. And to set the opening act of your WWII resistance romance in Casablanca could be taken as homage, but when a key plot point involves someone defiantly playing the Marseillaise in front of Nazis, you’re virtually breaking the fourth wall when it comes to your reference points. With that, and an oddly soundstage-y feel to many of the locations, it means that you might be engaged in what’s going on, but you never really believe it.

When Zemeckis puts the bells and whistles down and focuses on making a character-driven thriller in an almost classical form, “Allied” is the most engaging thing he’s made since “Contact.” That we sound like we’re beating up on it is mostly a reflection of how close it comes to being very good, and how frustrating it is that it falls wide of the mark. We really wanted to love “Allied,” and that we came away sort-of-liking-it-I-guess just isn’t good enough in the end. [C]