[2] In 1943, Naval intelligence officer Ewen Montagu welcomes the audience to MI5, where the "elite" of the British military contribute to the World War II effort ("Born to Lead").

Colonel Johnny Bevan informs his subordinates that the British government requires a strategy to convince the Nazis that the Allied Forces are planning to invade Sardinia rather than their actual (and more obvious) target, Sicily; several officers, including Montagu and Ian Fleming, present their ideas, but despite their conviction of their own brilliance ("God That's Brilliant"), Bevan dismisses them all as uncreative and poorly planned.

Montagu reads his plan, and despite declaring it "insane", recognises its potential and convinces Charles that they should team up to pitch Mincemeat to Bevan, noting his own natural "flair for presentation" ("Dead in the Water (Reprise)").

Meanwhile, secretary Hester Leggatt welcomes new clerk Jean Leslie to her team, who disdains the typically side-lined role of women in MI5 and dreams of ranking alongside the male officers ("All the Ladies").

That night, Montagu and Charles arrive in Scotland to load "Bill's" corpse onto the submarine HMS Seraph; its crew are told that the sealed canister contains meteorological equipment ("Sail On, Boys").

Charles is deeply insecure about the result of the operation, but is swayed by Montagu into celebrating its completion, later witnessing a brief meeting between the latter and Ivor ("Just For Tonight"); meanwhile, after enduring enemy attacks, the crew of the Seraph are unnerved by the contents of the canister but follow their orders, placing Bill in the sea beyond the Spanish coast before thanking him for his service.

The team await news from Vice-Consul Francis Haselden in Huelva, but are dismayed to learn that the victim of an aeroplane accident he calls to report is a live American soldier, also named William; they fear it will make Bill’s story less believable.

Moments later, they learn Bill has also been recovered, and they instruct Haselden to make sure the briefcase holding the false plans is seized by the Nazi-populated Spanish authorities, amidst news that the latter are indeed suspicious and have ordered an autopsy ("The Ballad of Willie Watkins").

An update from Bevan then confirms that Haselden's whisper campaign has worked, and Adolf Hitler himself has ordered the transfer of German troops from Sicily to Sardinia.

The Allies subsequently invade Sicily with little-to-no bloodshed, and Bevan informs an ecstatic Montagu, Charles, Jean and Hester that Operation Mincemeat is officially a success, and that they will receive the highest level of commendation in the British Army ("Did We Do It?").

To counteract this, they relate a summary of everything that occurred following the success of Mincemeat: the Second World War is won by the Allies with help from the US Army; Ian Fleming goes on to create James Bond; Haselden and his assistant Steve remain in Spain; Hester initiates a romance with Bevan; Jean looks ahead to the progress of feminism during and after the war; Montagu eventually makes (and acts in) his film about Operation Mincemeat; and Charles goes on to do classified work for MI5.

Keys The original West End cast recording for Operation Mincemeat was released on 12 May 2023, on streaming platforms and physically on CD and vinyl.