Director Robert Zemeckis is perhaps best known for his films involving special effects  (including Contact and Back to the Future), but Allied relies very much on the story of two people in World War II: Wing Commander Max Vatan (Brad Pitt), a Canadian  intelligence officer working with the British forces, and resistance fighter Marianne Beausejour (Marion Cotillard).

Writer Steven Knight’s dialogue and plot twists maintain the film’s dramatic tension.

Allied begins in 1942 with Vatan being parachuted into a desert in French Morocco. The Nazis have taken control of Casablanca and everything is set up for Vatan to meet with Beausejour and for them to begin the pretence that they are husband and wife.

They are skilled at their jobs and Beausejour, having lived in Casablanca for some time, has local knowledge: she comfortably speaks in French, English and German. She has also become a well-known identity and party girl in the city, which is under Nazi control although there are resistance fighters and spies are everywhere.

Inevitably, Beausejour and Vatan become attracted to each other and she convinces him to consummate their “marriage” in a small vehicle in the middle of a sandstorm.

In many spy thrillers, tension is created by the possibility of the spy slipping up with a little detail that may give away the entire operation; in Allied, it initially centres on whether Vatan’s accent, being more Québécois than Parisian, will be detected by the locals or the Germans.

After executing a bold and daring plan to assassinate a German ambassador at an important social function, the two escape, only to meet again in London, where they marry and have a child.  But after a year in a desk job, Vatan is called in by his superiors and faced with a terrible dilemma which forces him to question who he trusts – and the decisions he takes will have enormous implications for himself, his family and those around him.

Pitt and Cotillard create a believable attraction and develop a convincing, loving relationship: her character is alluring and seductive, but also determined and quite ruthless; his is handsome and attractive, but clever, capable and willing to take risks.

There is subtlety and tenderness in the actors’ performances and an enjoyable on-screen romance.  Their strengths lie in the moments when there are doubts in their relationship and they must face the consequences of those doubts.

Maintaining interest and intrigue until the final moments, Allied shows people who are trapped by the circumstances of war, and who are forced to do things against their will. It is a film about trust: who you trust and – of those closest to you – who you believe.

Allied opens in cinemas on Christmas Eve.