With Napoleon defeated and exiled, the reluctant Duke of Wellington is persuaded by Lord Castelreagh to represent Great Britain's interests at the Congress of Vienna, where the victorious allies will decide the future of Europe. While there, his friend the Duchess of Richmond introduces the married man to the pretty Lady Frances Webster, an ardent admirer, at her ball. During the course of the evening, however, Wellington receives an urgent message: Napoleon has escaped and has landed in France. French King Louis XVIII and his niece and most trusted adviser, Madame, the Duchess d'Angoulême, are not alarmed in the least. Ney, formerly one of Napoleon's marshals, volunteers to take 4000 picked men and capture his former leader. However, he switches sides. With France once again under Napoleon's control, both sides race to reassemble their armies. Napoleon routs the Prussians under Marshal Blücher before coming to grips with his old nemesis Wellington at the Battle of Waterloo. At the crucial point of the battle, Blücher's timely arrival turns the tide, and Napoleon is defeated for the final time. The allies occupy France and gather in Paris to divide the spoils. Once again, Castelreagh sends Wellington to try to restrain the others from punishing France too severely, in order to ensure a lasting peace. Wellington's task is made more difficult by the opposition of Madame, who is certain he wants to rule France himself. Wellington warns Louis that Madame's desire to have the still popular Ney executed for treason would risk another revolution. Madame arranges for Wellington's recall to London, to answer a newspaper story that he is carrying on an affair with Lady Frances. Wellington soon disproves the claim, but while he is gone, Ney is convicted and shot by firing squad. The French people are outraged. Upon his return, Wellington forces the King to dismiss his advisers, including Madame. Back in London, Wellington has to defend his decision to accept no reparations for his country.
The film was the ninth most popular at the British box office in 1935–36. The New York Times wrote,
The Iron Duke can be recommended to Mr. Arliss's admirers everywhere as a pseudo-historical drama which manages to be both impressive and amusing and which reveals the star at his very best... The film, surprisingly enough, is not at its best in its dramatization of Waterloo, which has been so simplified that it seems a rather placid affair on the screen. It is in such scenes as Wellington's appearance before the House of Lords to answer his defamers and his broken-hearted demeanor as he scans the casualty lists after the battle that the drama becomes genuinely stirring. Since this is Mr. Arliss's Wellington, the authors have arranged numerous dialogic whimsies to illustrate the conqueror's benign and gentle humors. Neither Gladys Cooper nor A. E. Matthews has much to do in the film. The lengthy cast which assists the star includes a number of capable players;
while more recently, TV Guide gave the film two out of four stars, and wrote, "Not only are the pace and direction of The Iron Duke uninspired and haphazard, but the script is rife with historical inaccuracies, the glossing over of less flattering events, and definite misrepresentation in the case of Marshal Ney's execution"; and Britmovie called the film a "colourful yet flat historical drama", though it praised George Arliss, "who was skilled at playing historical characters and delivers a typically perceptive performance."
