In 1902, British aristocrat Orlando (Ralph Fiennes) (Arthur), Duke of Oxford, his wife Emily (Alexandra Maria Lara), and their young son Conrad (Harris Dickinson) visit a concentration camp (led by General Kitchener and his deputy Captain Maximilian "Max" Morton) in South Africa during the Second Boer War while working for the Red Cross. Emily is mortally wounded during a Boer sniper attack on the camp. Before she dies, Emily makes Orlando promise never to let their son see war again.Twelve years later, Orlando has formed a private spy network consisting of domestic servants employed by the world's most powerful dignitaries and has recruited his own servants Shola (Djimon Hounsou) (Merlin) and Polly (Gemma Arterton) (Gallahad) into it. The network's primary objective is to protect the United Kingdom and the British Empire from the approaching Great War. Conrad is eager to fight, but Orlando forbids him to join the British Army and persuades Lord Kitchener (Charles Dance), Secretary of State for War, not to let him do so.At the behest of Kitchener, Conrad and Orlando ride with Orlando's friend Archduke Franz Ferdinand (Ron Cook) through Sarajevo. Conrad saves the Archduke from a bomb thrown by Gavrilo Princip (Joel Basman), a rebel intent on sparking a war. Princip later encounters the Archduke's entourage again by chance and succeeds in fatally shooting him and his wife Sophie.King George of England, Tsar Nicholas of Russia and Kaiser Wilhem of Germany were cousins, kept in check by Queen Victoria. The death of Archduke Ferdinand will allow Kaiser Wilhem to move his armies against Austria and disturb a fragile peace. On Shepherd's orders, Erik, an advisor to Kaiser, goads him to ignore King George and attack Austria. This quickly escalates into WW I and weakens Europe.Orlando visits the prison in Sarajevo and learns that Princip is part of "The Flock": a group plotting to pit the German, Russian, and British empires against each other. The Flock has its headquarters on an isolated clifftop and is led by "The Shepherd" Captain Maximilian "Max" Morton (Matthew Goode) (who is still working under Kitchener at the British War ministry), whose ultimate goal is to achieve independence for Scotland; among its operatives is Russian mystic Grigori Rasputin (Rhys Ifans), trusted adviser (The Tsar's personal physician) to Tsar Nicholas of Russia.On the Shepherd's orders, Rasputin poisons Tsar Nicholas's young son Alexei, only to cure him when Nicholas promises to stay out of the war. Conrad is notified of Rasputin's manipulation by his cousin, Prince Felix Yusupov (Aaron Vodovoz). Knowing the Western Front will be left vulnerable if Russia leaves the war, Conrad delivers this information to Kitchener and his aide-De-camp Major Max Morton, who set sail for Russia. Their ship is torpedoed by a submarine and sunk.Word of Kitchener's death reaches Orlando, spurring him (to first induct Conrad into his secret spy organization) to travel to Russia with Shola, Polly, and Conrad to deal with Rasputin once and for all. After a grueling close fight, Orlando, Shola, Conrad, and Polly successfully kill Rasputin during a Christmas party hosted by Prince Felix. Later that night, after celebrating his 19th birthday, Conrad expresses his determination to join the army, much to Orlando's dismay.At the Shepherd's orders, Erik Jan Hanussen (Daniel Bruhl) (An occultist and the Shepherd's second-in-command), an adviser to Kaiser Wilhelm II, sends the Zimmermann Telegram (German Foreign Minister Arthur Zimmermann to the German Minister to Mexico, Heinrich Von Eckhardt, offering United States territory to Mexico in return for joining the German cause), hoping to distract Britain and the United States. Although the message is intercepted by British intelligence and deciphered by Polly, President Woodrow Wilson refuses to join the war, citing a lack of concrete proof. The Shepherd recruits Vladimir Lenin and orders his Bolsheviks to overthrow the Tsar and remove Russia from the war, sending an assassin to kill the Romanovs.Now of age, Conrad is commissioned into the Grenadier Guards against his father's wishes, but Orlando persuades King George V (Tom Hollander) (Percival) to have him assigned to London. Determined to fight in the war, Conrad swaps places with a soldier named Archie Reid (Aaron Taylor-Johnson), giving him the nickname "Lancelot" to send his father a message. Disguised as Archie, a member of the Black Watch, Conrad volunteers for a mission into No-Man's Land to retrieve information from a British agent wounded there. He is mistaken for a German spy (he gives his name as Archie Ried, but Archie mates say that they know Archie, and he ain't him) upon his return and executed at gunpoint, devastating Orlando, but the information he has recovered verifies the authenticity of the Zimmermann Telegram.After Wilson again refuses to enter the war despite Conrad's proof, Orlando learns that Wilson is being blackmailed with film footage showing him being seduced by the Shepherd's agent, Mata Hari (Valerie Pachner), and resolves to recover it. Upon defeating Hari at the American embassy, Orlando has her cashmere scarf identified as being made from rare wool only found in a specific mountainous region. Pinpointing this location as the Shepherd's base of operations, Orlando, Shola, and Polly head there and fight their way inside. The Shepherd is revealed to be Morton, who had faked his own death and killed Kitchener himself. Orlando and Shola fight and kill Morton while Polly recovers the film; it is delivered to Wilson, who burns it and prepares to mobilize American troops.A year after the war, Orlando purchases the Kingsman Tailor Shop as a front for his organization. Orlando, Shola, Polly, King George, Archie (Lancelot), and the U.S. Ambassador to the U.K. (Bedivere form the original Kingsman, each assuming code-names from the King Arthur legend to honor Conrad. In a mid-credit scene, Hanussen, having taken on the Shepherd identity, introduces Lenin to the Romanovs' killer: a young Adolf Hitler.