In the midst of World War II in 1943, the United Kingdom finds itself engulfed in conflict. Lieutenant Commander Ewen Montagu (Colin Firth), a Jewish lawyer, remains in England as his wife, Iris (Hattie Morahan), and their children seek refuge in the United States. Ewen is burdened by the fear that, should Hitler reach London, his family would be in grave danger. Compounding his worries, Iris contemplates a divorce, believing that if Ewen truly cared, he would join them in the U.S.
Taking a break from his legal career, Montagu is enlisted to serve on the Twenty Committee, disguised as a nautical supply officer. Accompanying him is his dedicated secretary, Hester Leggett (Penelope Wilton).
Winston Churchill (Simon Russell Beale) has committed to the United States that the Allies will launch an invasion in Sicily by July. However, as Sicily is deemed an obvious target, the risk of heavy German resistance looms large. Admiral Godfrey (Jason Isaacs) briefs the committee on the necessity to mislead Nazi Germany into thinking the Allies intend to invade Greece instead. Charles Cholmondeley (Matthew MacFadyen) suggests a daring plan outlined in the Trout Memo, involving a corpse with fake intelligence that will wash up on the shores of Spain, under German control. Despite Godfrey's skepticism about the plan’s feasibility, he allows Montagu and Cholmondeley to develop the details, with assistance from Lieutenant Commander Ian Fleming (Johnny Flynn).
As they strategize, several potential pitfalls arise. The body might decay upon being dropped from the air, or an autopsy could reveal foul play. Furthermore, there's a possibility that the briefcase's contents may be returned unopened by the Spanish authorities.
Operation Mincemeat begins in earnest. Montagu and Cholmondeley acquire the body of a deceased vagrant named Glyndwr Michael (Lorne MacFadyen), whose death was likely a result of suicide by poisoning. They fabricate a backstory for him, claiming he is Major William Martin of the Royal Marines. They even create a full identity complete with photographs and an engagement to Jean Leslie (Kelly Macdonald), who, as a secretary in the office, contributes her photo to this deception. Cholmondeley develops feelings for Jean, only to find himself grappling with jealousy as he becomes aware of the budding romance between Montagu and Jean.
Amidst this personal turmoil, Admiral Godfrey suspects Montagu's brother, Ivor (Mark Gatiss) is feeding information to Russian spies. He coerces Cholmondeley into spying on Montagu with the promise of retrieving the remains of his own brother, who died in action in Bengal.
A decision is made to transport the corpse by submarine rather than by air to prevent disintegration upon impact. Detailed discussions about the plan ensue with Salvador Gomez-Beare (Will Keen) and Captain David Ainsworth (Nicholas Rowe). Their efforts to ensure that the critical documents reach the Nazis are further complicated by identifying two German spies in Spain.
As Cholmondeley uncovers Ivor’s true affiliations as a spy, he confronts Jean with troubling revelations about Montagu’s familial situation, leaving her heartbroken and confused. Meanwhile, St John "Jock" Horsfall (Mark Bonnar) drives Montagu, Cholmondeley, and the corpse to a submarine base, where the body is loaded onto HMS Seraph, commanded by Lt. Bill Jewell (Rufus Wright). In a surprising turn, Cholmondeley decides he must accompany the mission, provoking a confrontation with Montagu. In a moment of vulnerability, Montagu confesses his love to Jean.
On April 30, the Seraph drops the corpse into the Gulf of Cádiz, where fishermen locate it shortly after. As the operation unfolds, unforeseen obstacles arise with the Spanish authorities showing unusual resistance to Nazi influence, complicating the plans to deliver the fabricated documents.
In an attempt to salvage the operation, Captain Ainsworth engages Colonel Cerruti of the Spanish secret police to ensure the papers reach the Nazis, but the stakes are high. Meanwhile, Montagu learns of Jean's encounter with a man who poses a threat to their plan. As tensions mount, Montagu shares safe haven with Jean until she takes a position within Special Operations, leading her away from London.
By July 10, the Allies engage in the Sicilian invasion, with news filtering in of only limited casualties and successful territory retention. Following this crucial victory, Cholmondeley reveals his own sacrifices in spying for Montagu, which leads to a moment of reconciliation over drinks—albeit early in the day.
In the aftermath, life returns to a semblance of normalcy. Montagu and Iris ultimately reunite post-war, Jean moves on and marries a soldier, while Hester remains at the Admiralty. Cholmondeley continues with MI5 until 1952. The identity of the deceased is confirmed as Glyndwr Michael, earning recognition years later when his real name appears on his tombstone in Spain. The echoes of Operation Mincemeat resonate in history as a bold act of deception that played a pivotal role in altering the course of the war.
