Operation Mincemeat, review: a businesslike account of one of history's most outrageous military cons

Tricking the Nazis into a troop withdrawal from Sicily – just before the Allies mounted the largest amphibious invasion there in history – needed one corpse, planted to save thousands, and an awful lot of head-scratching.

This infamous ruse is retold in John Madden’s squarely watchable Operation Mincemeat, a cram-it-all-in adaptation of Ben Macintyre’s 2010 history book of the same name, which knuckles down to its task with sleeves rolled, upper lips stiffened, and vast sheaves of exposition to whip through.

The story hinges on wild unlikelihoods. Only something fantastically improbable was capable of fooling the wily minds of Axis intelligence – namely, the ploy to stuff a cadaver with papers identifying the dead man as a British serviceman, along with forged documents “revealing” the invasion was actually to take place in Greece, and to float him off the southwestern coast of Spain, near Cadiz.

Much of Madden’s businesslike spy drama, with its insistently witty Michelle Ashford script, is spent arguing the toss about just how likely the Nazis would be to swallow all this. If they fell for it, the Sicilian assault favoured by Churchill (a never-plummier Simon Russell Beale) could proceed with way fewer casualties.

As we begin, the idea of dangling this bait drops down from something called the Trout Memo, issued under the name of Admiral John Godfrey (Jason Isaacs, on brutally sardonic form), but thought by Macintyre to have been penned instead by Godfrey’s assistant, one Lieutenant Commander Ian Fleming (a drawling, slightly dubious Johnny Flynn).

Fleming’s back-seat role, smoking at his typewriter and tipping his hat to favourite spy writers who preceded him (such as Basil Thomson), teases us with a pre-007 origin story. He calls Godfrey “M”, and even pops down to “Q branch” at one point – chief supplier of gadgets to the Secret Service – where a watch functioning as a rotating buzzsaw puts a gleam in his eye.

This is a jolly sideshow. The star players in the saga are mostly desk jockeys: the counter-espionage team who set Mincemeat in motion and chewed their fingernails while it nearly came apart.

This is the so-called Twenty Committee (the Roman numerals XX connoting a double-cross), their efforts overseen by Captain Ewen Montagu (Colin Firth, on the dry side here), a Jewish barrister whose family were out of harm’s way in the US. His second-in-command is Charles Cholmondeley (Matthew Macfadyen), a tall, eccentric RAF officer whose eyesight kept him grounded, a self-described “flightless bird”. Thanks to Macfadyen’s high-IQ subtlety, he’s the most engaging figure by a distance.

This prickly pair were the main architects of the plan, with help from secretarial head Hester Leggett (Penelope Wilton) and her protégé Jean Leslie (Kelly Macdonald). It was the latter who agreed to provide a seaside snap of herself to give the fictitious dead serviceman, Major William Martin, an equally fictitious sweetheart called Pam.

This is a lot, but barely the half of it. Madden and Ashford have a huge amount to cover in two hours: what Spanish officials did with the body when it actually washed up might have been a film unto itself, as might some bisexual intrigue in Cadiz with an agent called David Ainsworth (Nicholas Rowe), whose connections were vital to keep the ruse on track.

To convince us of an urgently ticking clock, the third act involving all this gets rushed, with Thomas Newman’s score elbowing it along. There were other ways to buy time. A burgeoning Firth/Macdonald affair, animated without much spark, and Macfadyen’s jealousy on the sidelines, probably shouldn’t have bogged us down for such long, rather drowsy stretches through the middle.

Madden has done sprier directing jobs – indeed, we’re generally garrisoned in a square quarter-mile somewhere near The Mall, with good actors declaiming an awful lot of lines back and forth on the pavement, and the emotional heft of the story underlined by ruler and marker pen.

The fact that we now know the identity of the corpse, a homeless Welshman called Glyndwr Michael who drank rat poison to kill himself, seemingly obliges the film to lean on tributes to the heroism of the unwitting dead.

Lighter touches from the ensemble are never unwelcome. A voiceover by Flynn is molten lead, though, poured in at the start and end like a radio announcement for dour respectability. Concurrently, there’s a fringe show running with the same name – a musical spoof touted as funny, ingenious and touching. It’s proof that the hand of history needn’t always be the headmasterly shoulder-clasp this is.

In cinemas now. 12A cert, 128 min