Set against both 1930s Nazi Germany and the present day, the novel interweaves two storylines: a Munich police detective's decades-long fight to expose Adolf Hitler's murder of his half niece, and a modern historian's dangerous quest to recover the proof he left behind.
The historical narrative opens in September 1931 at a Munich cabaret, where Hitler attends with his twenty-three-year-old half niece, Angela "Geli" Raubal. After Geli delivers a stunning vocal performance, Hitler is furious at being upstaged and takes her home to his flat at Prinzregentenplatz 16. He demands she submit to a degrading sexual routine. Geli refuses and announces she is leaving for Vienna to marry her vocal instructor, a Jewish man, and reveals she is pregnant. Hitler punches her, breaking her nose; in the struggle, Geli briefly seizes his Walther pistol but cannot fire. Hitler wrests the gun back and shoots her in the chest, killing her. He phones his personal secretary, Rudolf Hess, for help.
The present-day narrative introduces Eleanor Robbins, a fifty-eight-year-old history professor from the University of Chicago who is blind in one eye and grieving her daughter's death from cancer. A man named Francis Linderman, whose family had preserved documents connected to the original Nazi-era investigation, sent Eleanor research materials, including two journals from the Nazi era, before his death, which was ruled a suicide. The materials describe a plot to murder Hitler in 1945 that allegedly succeeded. Eleanor travels to Europe to find corroborating proof.
Back in 1931, Detective Inspector Thomas Bauer, a decorated World War I veteran, arrives at Hitler's flat to investigate Geli's death. He confronts Nazi brownshirts blocking his entry and finds Hess inside insisting Geli died by suicide before any examination has been performed. Bauer's inspection reveals blood spatter, shards from a hastily cleaned-up broken mirror, and a chest wound rather than a head shot, all indicating a staged scene. When he interviews the housekeepers, the wife begins to confirm Hitler was present that night, but Chief Inspector Heinrich Müller shuts down the interview and removes the case from Bauer's jurisdiction. The body is moved on orders of the Bavarian justice minister before a proper autopsy, and the official report falsely rules suicide. The medical examiner privately tells Bauer the broken nose came from a blow, the wound range was too far for self-infliction, and the bullet trajectory was downward.
Hitler's inner circle manages the cover-up. Joseph Goebbels, the propaganda chief, coins "unsatisfied artistic achievement" as Geli's supposed motive. Heinrich Himmler, head of the SS, the Nazi paramilitary organization, raids critical newspapers, and four Munich publications are firebombed. Goebbels deceives a family priest into allowing Geli's burial on consecrated Catholic ground by blaming her death on political enemies.
Bauer continues investigating off the books, allying with Fritz Gerlich, a conservative journalist and fierce Hitler critic. They compel Emil Maurice, Hitler's former chauffeur, to reveal the nature of Hitler's sexual acts: Hitler forced Geli to undress while he lay on the floor, demanded she squat over him, and insisted she urinate on him, acts she found revolting. Maurice also confirms that Hess and his assistant, Martin Bormann, cleaned the crime scene, unpacked Geli's suitcase, and removed the broken mirror.
Bauer breaks into the Brown House, Nazi Party headquarters, to steal documents from Hitler's personal safe, opening it with the combination 8-11-23, the date of the Beer Hall Putsch, Hitler's failed 1923 coup attempt. The envelopes contain Hitler's pornographic sketches of Geli, memos from Goebbels and Hermann Göring, one of Hitler's most powerful deputies, outlining plans to seize total power and exterminate Jews, Geli's unfinished letter cut off mid-word, and Hitler's graphic letter to Geli bearing his ink-stained thumbprint. Bauer's young assistant, Peter Helldorff, betrays them after the Nazis threaten to end his police career. The documents are recovered, and Bauer and Gerlich are arrested but released because the Nazis want no publicity.
In the present-day storyline, Eleanor faces escalating danger. Her hotel room is ransacked with "88," a neo-Nazi code for "Heil Hitler," spray-painted on the wall. In Vienna, a neo-Nazi operative warns her to abandon her research and reveals Francis Linderman was murdered. Eleanor briefly obtains the original letter with Hitler's thumbprint through an intermediary but has it snatched at gunpoint. She deduces that a sixth envelope, secretly added to the stolen bag by an unknown party, contained a reel of film that Helldorff kept as insurance before returning the documents to the Nazis.
The historical narrative advances through the 1930s. After Hitler becomes chancellor in January 1933, the Nazis orchestrate the Reichstag fire, blame the Communists, and use emergency decrees to suspend all constitutional rights. Gerlich is arrested and sent to the Dachau concentration camp. Bauer's Jewish father-in-law, Selig Geiger, is publicly beaten and humiliated. Bauer's wife, Amalie, is fired under anti-Jewish civil-service laws and detained. Bauer frees her and other detainees, then sends his family across the Austrian border, staying behind to fight. Gerlich is executed at Dachau on the Night of the Long Knives in June 1934. Helldorff, now considered a liability, is also killed that night but has left the film reel with his wife for delivery to Bauer.
Bauer and his ally Erich von der Haas, a former signals-intelligence officer and member of a resistance group called the Ten Patriots, spend over a decade attempting to assassinate Hitler. A bomb planted at the Bürgerbräukeller, the Munich beer hall hosting Hitler's annual putsch-anniversary speech, detonates on time in 1939 but Hitler leaves thirteen minutes early. A sniper attempt at Berchtesgaden in 1940 goes awry and Bauer is nearly killed. A 1942 shot in Nuremberg hits a body double. The war also devastates von der Haas personally: His brother Lars, a physics professor forced from his position, is deported with his family to the Sobibór extermination camp in Poland, where Lars's wife, daughter, and parents are killed in the gas chambers. Von der Haas vows to kill Hitler himself.
In April 1945, Bauer and von der Haas infiltrate the Vorbunker, the upper level of Hitler's bunker beneath the Reich chancellery, posing as supply corporals. On April 30, Hitler stages a fake suicide, arranging for his cook, who closely resembles him, to be burned in the chancellery garden alongside the body of Eva Braun, his longtime companion. Hitler walks to the Tiergarten airstrip, where a small plane awaits. Von der Haas, who has knocked out the pilot, confronts Hitler from the cockpit, naming each murdered family member, but cannot execute a defenseless man. When a nearby bomb blast distracts von der Haas, Hitler draws a concealed pistol. Bauer shoots him in the right temple. The two men escape Berlin in the plane.
In the present-day conclusion, Eleanor meets Franklin Linderman, Peter Helldorff's elderly son, in Munich. Franklin confirms the sixth envelope contained a film reel but is killed by a sniper during their conversation. His bodyguard, Jenny, rushes Eleanor to the airport. There, Eleanor's phone recording reveals that Jenny slipped the film reel into an interior pocket of Eleanor's corduroy jacket, a gift from her late daughter, when Jenny bumped into Eleanor at St. Stephen's Cathedral days earlier. Eleanor has been carrying the evidence all along.
In the epilogue, Eleanor views the film under FBI protective custody in Washington, DC. It contains footage from a hidden camera in Geli's bedroom showing the degrading acts witnesses described, and a final segment capturing the night of Geli's death: the argument, the punch, Geli seizing the gun, and Hitler shooting her. Eleanor prepares to present the film to experts from the National Archives, the Library of Congress, the Smithsonian, and the FBI for authentication, offering proof that Hitler murdered his half niece and that everything Thomas Bauer fought to expose was true.