The film tells the story of “Operation Mincemeat” in 1943, a famous Allied deception maneuver during the Second World War. While the British forces are preparing to invade Sicily, intelligence officers Ewen Montagu and Charles Cholmondeley want to mislead the Germans with false information. They devise a plan to take a corpse with forged documents into the sea off the Spanish coast. This corpse, disguised as a British officer, carries forged plans to simulate an invasion of Sardinia and the Peloponnese. In this way, the British hope that the Germans will concentrate their defenses on these regions and leave the real target of Sicily unguarded.
The Director of Intelligence, Admiral John Godfrey, is initially skeptical, but Montagu finally implements the plan. The corpse of a homeless man becomes “Major William Martin”, and the deceptively real documents in his clothing help the Germans to believe the scenario and shift their defense forces. The operation is ultimately a complete success, as the Germans take the bait and the Allied attack on Sicily is met with less resistance.