The final portion of Alexandre Dumas’s d’Artagnan Romances tells the story of Louis and Phillipe (Louis Hayward), the twin sons of France’s king Louis XIII (Albert Dekker). Upon their birth, Prime Minister Colbert (Walter Kingsford) conceals the news of twins and sends infant Phillipe to Gascony to be raised in secret by the loyal musketeer D’Artagnan (Warren William). Decades later a grown Louis is now a tyrannical king, aided by the wicked Minister of Finance Fouquet (Joseph Schildkraut). When he learns of his twin, he at first uses him as a double, which allows Phillipe to fall in love with Louis’s fiancée, Maria Theresa (Joan Bennett), but eventually Louis realizes Phillipe is a threat and has him imprisoned in the Bastille, wearing the infamous iron mask of the title and prompting D'Artagnan and the other musketeers to once again come to the rescue.