In London, in the spring of 1943, British Naval Intelligence ponders how to decoy German forces from the island of Sicily so that the British can launch their invasion there.  To accomplish this, Lieutenant Commander  Ewen Montagu devises an ingenious scheme to make the Germans believe that the British are deploying troops to Greece, hoping to lure the Germans there from Sicily.  To trick the Germans, Ewen plans to use a dead body, dressed in an officer's uniform and carrying top secret documents ordering the invasion of Greece.  When Montagu and his assistant, Lieutenant George Acres, decide to make the body appear as if it were drowned following an air crash at sea, a doctor advises that a victim of pneumonia would appear to have drowned because of the water accumulated in the lungs.

Dubbed "Operation Mincemeat," the strategy slowly develops to float the body off the coast of Spain where the current will carry it to shore.  The danger is that if the plan fails, the Germans will know that Sicily is the target of the invasion.  Montagu impatiently waits permission from the top brass until Prime Minister Winston Churchill finally gives the go-ahead.  The project hits a snag, however, because no suitable body can be found until Montagu convinces the grieving father of a Scotsman to allow him to use his dead son's pneumonia-wracked body for the good of Britain.

The next step is to fabricate an identity for the young man, whom they christen Major  William Martin.  Deciding that a love letter and a photograph of Martin's fiancée would create a touch of authenticity, Montagu asks his secretary Pam to write the letter, which will be slipped in Martin's wallet.  Pam is helped by her lovelorn roommate, Lucy Sherwood, whose fiancé Joe, a fighter pilot, has just left for combat.  Proceeding to the morgue, Montagu and Acres plant Lucy's photo and letter in Martin's wallet, along with several other personal documents, then meticulously dress the body and attach a briefcase bearing the secret documents to his wrist.  They then place the body in a refrigerated canister labeled "optical instruments" and transport it to the naval base.  There the canister is loaded aboard a submarine and transported to the Spanish coast, where the corpse is set adrift.

After the body washes ashore, it is found by fishermen, and Spanish police then notify the British Vice-Consul about the death of Major  Martin.  After the "major" is given a military funeral and interred in Spanish soil, his briefcase is returned to Britain, where all the documents appear to be intact.  Montagu fears that his mission has failed until a scientist examines the sealed papers and declares that they have been opened and photographed.

In Germany, meanwhile, an eager Hitler proclaims that the photocopied documents are genuine, but German Intelligence remains skeptical and so sends an agent to verify their authenticity.  The German, posing as Irishman Patrick O'Reilly, arrives in London, with a radio transmitter hidden in the bottom of his suitcase along with the copies of Martin's papers.  O'Reilly's first stop is the men's wear shop whose address appeared on a bill for shirts found in Martin's wallet.  After ascertaining that the shop sells the kind of shirts worn by Martin, O'Reilly scrutinizes a bank overdraft made out to Martin and phones the bank, claiming to be Martin's representative.  The bank manager, who has been alerted to the plan, promptly notifies Montagu about the call, who turns to Scotland Yard for help.

Lucy, meanwhile, learns that Joe has been killed in combat and goes into a state of shock.  O'Reilly next goes to Lucy's apartment but finds Pam there instead.  Posing as Martin's boyhood friend, O'Reilly asks to see Lucy.  Just then, Lucy comes home drunk and when O'Reilly questions her about her fiancée, she replies he is dead and therefore never existed.  After Lucy breaks down in tears and begins to ramble incoherently, O'Reilly gives her his address in case she is need of consolation.  After he leaves, Pam passes the address onto Montagu, who alerts Scotland Yard.

The cunning O'Reilly, meanwhile, contacts his superiors with the news that he has given his location to the enemy to see if they will come to arrest him, a sure sign that the Martin story is fictitious.  As Montagu and the men of Scotland Yard speed to O'Reilly's address, Montagu realizes that it is a set-up and convinces the officers to allow O'Reilly to escape.  When the police fail to appear, O'Reilly confirms that Martin is genuine and the Germans dispatch their troops to Greece, clearing the way for the British invasion of Sicily.

Awarded a medal for service to his country, Montagu travels to Martin's resting place in Spain and places it on his grave.