In 1943, the United Kingdom is deeply entrenched in World War II. Lieutenant Commander Ewen Montagu (Colin Firth), a Hebrew lawyer, remains in England while his wife, Iris (Hattie Morahan), and children travel to safety in the United States. Montagu worries that if Hitler reaches London a Hebrew family would be on the hit list. But Iris also plans a divorce from Montagu as she feels if he really cared, he would come to US with them. Montagu takes a hiatus from practicing law when he is appointed to the Twenty Committee (under the garb of serving as a nautical supply officer). His loyal secretary, Hester Leggett (Penelope Wilton), comes with him.Winston Churchill (Simon Russell Beale) has promised the United States that the Allies will invade Sicily by July of that year in order to push northward into Europe. However, Sicily is considered an obvious target and may be defended by the Wehrmacht. Admiral Godfrey (Jason Isaacs) informs the Twenty Committee that Britain must trick Nazi Germany into believing the Allies will invade Greece. Charles Cholmondeley (Matthew MacFadyen) proposes an operation from the Trout Memo, which would entail a corpse carrying false secrets and washing ashore in Spain (which is under German occupation). Despite Godfrey's doubts (he feels this is a simple deception and the Nazis would see right through it), he gives Montagu and Cholmondeley permission to plan the operation with Lieutenant Commander Ian Fleming (Johnny Flynn). The plan has many loopholes. The body could disintegrate on impact (when thrown from the air). Autopsy could reveal that the person didn't die from drowning. Also, the contents of the briefcase could be returned by the Spanish, without ever being opened.Operation Mincemeat sets up an office. As planning gets underway, Montagu and Cholmondeley obtain the body of a vagrant named Glyndwr Michael (Lorne MacFadyen), who died by possible suicidal poisoning. The team gives him the false identity of Major William Martin, Royal Marines, complete with a detailed backstory, ID photos (Photographs taken of a person who looks just like Glyndwr), and an engagement. A widowed secretary in the office, Jean Leslie (Kelly Macdonald), offers a photo of herself to serve as the fake fiance. Cholmondeley has a crush on Jean, but soon realizes that Montagu and Jean have romantic feelings for one another. This causes Cholmondeley to grow jealous and occasionally lash out at Montagu.Godfrey suspects Montagu's brother, Ivor (Mark Gatiss), is a spy for Russia. He bribes Cholmondeley to spy on Montagu and, in return, Godfrey will locate and return the remains of Cholmondeley's brother, who was killed in action in Chittagong, Bengal. Cholmondeley reluctantly agrees.Decision is made to deliver the dead body by submarine, rather than by air thereby avoiding disintegration. The minute details of the plan are discussed with the British Naval attache in Spain Salvador Gomez-Beare (Will Keen) & Captain David Ainsworth (Nicholas Rowe), once the body arrives there with the secret papers. 2 German spies are identified who are key to finding the papers, thereby making sure that the contents of the briefcase are communicated to Berlin. In Berlin Col Von Roenne is the head of German intelligence. For anything to make its way to Hitler, the information would have to be endorsed by Von Roenne, who is a brilliant spy. But Von Roenne is suspected to be anti-Hitler.Meanwhile Cholmondeley confirms that Ivor is a spy for the Russians but has not yet revealed the existence of Operation Mincemeat to the Russians. Cholmondeley reveals to Jean that Montagu is married and that his wife and 2 kids live in England. Jean is devastated and confronts Montagu. Specialist MI5 driver St John "Jock" Horsfall (Mark Bonnar) transports Montagu, Cholmondeley, and the corpse to the R.N. Submarine Base in Holy Loch. The corpse is then loaded onto the submarine HMS Seraph, under the command of Lt. Bill Jewell (Rufus Wright). At the last-minute Cholmondeley changes the plan and decides to ride on the Seraph to Spain. This angers Montagu. Montagu goes to Jean and professes his love for her.On the morning of 30 April, the Seraph arrives in the Gulf of Cádiz and drops the corpse into the ocean. It is found by fishermen in Huelva, Spain. Operation Mincemeat attempts to get the fake documents to Madrid. However, the mission is hampered by bad luck, as the Spanish have resisted Nazi corruption better than expected. The Spanish navy summons Salvador Gomez-Beare and offers to return the briefcase to him, without routing them through the Nazis. Captain David Ainsworth (Nicholas Rowe), the British naval attache in Madrid, meets with Colonel Cerruti of the Spanish secret police (who is known to a fascist and devoted to Hitler) in one last attempt to get the papers to the Nazis. When Martin's personal things are returned to London, a specialist at Q Branch figures out that the documents were tampered with (The letters were folded in a particular manner before shipping and the fold patterns upon their return were analyzed and found to be different). This gives Operation Mincemeat hope that Germany retrieved the false information. Hester tells Jean that Iris and Montagu are separated for the moment.Jean is ambushed and threatened by Teddy (Jonjo O'Neill), a man claiming to be a spy for an anti-Hitler plot within Germany. Teddy knew Jean from the club which she frequented with Cholmondeley and Montagu. He wants to know why Jean's photograph was found on Martin's body. She tells him that Major Martin was travelling under an alias, but the classified information was genuine. He was carrying Jean's photo as it was stolen, and probably Martin was secretly having a crush on her. After Teddy leaves, Jean informs Montagu and Cholmondeley. They come to believe that Colonel Alexis Von Roenne, who controls intelligence in the Nazi High Command, sent Teddy to verify information (for it to be false) so Von Roenne could undermine Hitler by leading him down the wrong path. However, they have no way of being completely sure. Montagu takes Jean to his home for protection, but she accepts a job in Special Operations and soon leaves London.On 10 July, the invasion of Sicily commences. The news arrives that the Allied Forces suffered limited casualties, the enemy is retreating, and the beaches have been held. Afterwards, Cholmondeley admits he received his brother's remains in return for spying on Montagu. Feeling sympathetic and relieved that Operation Mincemeat was a success, Montagu offers to buy Cholmondeley a drink even though it's eight in the morning.The epilogue explains that Montagu reunited with Iris after the war, Jean married a soldier, Hester continued as Director of the Admiralty Secretarial Unit, and Cholmondeley remained with MI5 until 1952, later married, and traveled widely. Major William Martin's identity was revealed to be Glyndwr Michael in 1997 when an epitaph, with his real name, was added to Martin's headstone in Spain.