During World War II, Wehrmacht Colonel Claus von Stauffenberg (Cruise) is severely wounded during an RAF air raid in Tunisia, losing his right hand, the ring and little finger on his left hand, and his left eye, and is evacuated home to Nazi Germany. Meanwhile, Major General Henning von Tresckow (Branagh) attempts to assassinate Adolf Hitler by smuggling a bomb aboard the FÃ¼hrer's personal airplane. The bomb, however, is a dud and fails to detonate, and Tresckow flies to Berlin in order to safely retrieve it. After learning that the Gestapo has arrested Major General Hans Oster, he orders General Olbricht (Nighy) to find a replacement. After recruiting Stauffenberg into the German Resistance, Olbricht presents Stauffenberg at a meeting of the secret committee which has coordinated previous attempts on Hitler's life. The members include General Ludwig Beck (Stamp), Dr. Carl Goerdeler (McNally), and Erwin von Witzleben (Schofield). Stauffenberg is stunned to learn that no plans exist on the subject of what is to be done after Hitler's assassination.
During a bombing raid on Berlin, he gets the idea of using Operation Valkyrie, which involves the deployment of the Reserve Army to maintain order in the event of a national emergency. The plotters carefully redraft the plan's orders so that they can dismantle the Nazi rÃ©gime after assassinating Hitler. Realizing that only General Friedrich Fromm (Wilkinson), the head of the Reserve Army, can initiate Valkyrie, they offer him a position as head of the Wehrmacht in a post-Nazi Germany and request his support, but Fromm declines to be directly involved, stating he will not side with them so long as Hitler is alive. With the rewritten Operation Valkyrie orders needing to be signed by Hitler (Bamber), Stauffenberg visits the FÃ¼hrer at his Berghof estate in Bavaria. Fromm's influence allows Stauffenberg to bring the copy directly before Hitler, and in the presence of Joseph Goebbels, Heinrich Himmler, Hermann GÃ¶ring, Wilhelm Keitel and Albert Speer, his inner circle, Hitler praises Stauffenberg's heroism in North Africa and signs the orders without fully examining the modifications, believing Stauffenberg's changes "are for the best".
At Goerdeler's insistence, Stauffenberg is ordered to assassinate both Hitler and SS head Himmler at the FÃ¼hrer's command bunker, the Wolf's Lair. At a final briefing, Colonel Mertz von Quirnheim (Berkel) instructs the committee members in the use of pencil detonators. Stauffenberg also persuades General Fellgiebel (Izzard), who controls all communications at Wolf's Lair, to cut off communications after the bomb blast. On JulyÂ 15, 1944, Stauffenberg attends a strategy meeting at Wolf's Lair with the bomb in his briefcase, but with Himmler not present at the meeting, Stauffenberg does not get the go-ahead from the committee leaders, and by the time von Quirnheim defies the others and tells him to do it anyway, the meeting is over. Meanwhile, the Reserve Army is mobilized by Olbricht, unbeknownst to Fromm, to stand by. With no action taken, Stauffenberg safely extracts himself and the bomb from the bunker, and the Reserve Army is ordered to stand down, believing that the mobilization was a drill. Back in Berlin, Olbricht and Stauffenberg are threatened by Fromm that if they try to control the reserve army again he will personally have them arrested; Stauffenberg goes to the committee to protest their indecisiveness and condemns Goerdeler, who has been selected to be chancellor after the coup. When Goerdeler demands that Stauffenberg be relieved, Beck informs him that the SS has issued a warrant for his arrest, and that he must leave the country immediately.
On July 20, 1944, Stauffenberg and his adjutant Lieutenant Haeften (Parker) return to Wolf's Lair. To Stauffenberg's dismay, he discovers only after the timer has been activated that, due to the warm weather, the conference is being held in an open-window summer barrack, whereas the plotters had intended to detonate the bomb within the walls of the bunker for maximum damage. While his adjuntant waits with the car, Stauffenberg places the briefcase with the bomb armed at the meeting as close to Hitler as possible. Stauffenberg then leaves the barrack, returning to the car. However, one of the officers at the meeting moves the bomb behind a table leg, which will inadvertently protect Hitler from most of the blast. When the bomb explodes, Stauffenberg is certain that Hitler is dead and flees Wolf's Lair. Before shutting down communications, Fellgiebel calls Mertz about the explosion but cannot clearly convey whether or not Hitler is dead.
As Stauffenberg flies back to Berlin, Olbricht refuses to mobilize the Reserve Army until he knows without a doubt that Hitler is dead (if Hitler isn't dead, Olbricht will be arrested for having the reserve army mobilized without Fromm's permission). Behind Olbricht's back, Mertz forges his signature and issues the orders anyway. With Operation Valkyrie underway, Stauffenberg and his fellow plotters order the arrest of Nazi party leaders and SS officers, convincing lower officers that the Party and the SS are staging a coup. As Army soldiers begin to take control of Berlin's government ministries, including SS headquarters, mid-level officers relaying the orders begin to wonder which side they should be fighting for. Rumors reach Berlin that Hitler survived the blast, but Stauffenberg dismisses them as SS propaganda. Meanwhile, Fromm learns from Field Marshal Keitel that Hitler is still alive. The General refuses to join the plotters, resulting in them detaining him. Major Otto Ernst Remer of the Reserve Army prepares to arrest Goebbels, but is stopped when Goebbels connects him by phone to Hitler.
Immediately recognizing the voice on the other end, Remer realizes that the Reserve Army has been dupedârather than containing a coup, they have unwittingly supported it. SS officers are released and the plotters in turn are besieged inside the Bendlerblock. The headquarters staff flees, but the resistance leaders are arrested. In an ultimately vain effort to save himself, General Fromm convenes an impromptu court martial, places Beck under arrest, and sentences Von Quirnheim, Olbricht, Haeften, and Stauffenberg to death, contravening Hitler's orders that they be kept alive. Given a pistol by Fromm, Beck commits suicide. That night, the ringleaders are then executed by firing squad one by one. Some distance away, General Von Tresckow commits suicide with a grenade. When Stauffenberg is about to be shot, in a last gesture of loyalty and defiance, Haeften places himself in the path of the bullets meant for Stauffenberg. When his turn arrives, Colonel Stauffenberg's last act is to cry "Long live sacred Germany!"
A brief epilogue informs that the conspiracy of July 20, 1944, was the last of fifteen known assassination attempts on Hitler by Germans. It also mentions Hitler's suicide nine months later and that Countess Nina von Stauffenberg and her children survived the war. A shortened version of the dedication at the Memorial to the German Resistance is then shown onscreen:


You did not bear the shame
You resisted.
By sacrificing your impassioned lives,
For freedom, justice and honor.
Who is the only person that can initiate Valkyrie?
General Friedrich Fromm