The film starts with Mary Stuart, Queen of Scots, held prisoner in Fotheringhay Castle, awaiting the final judgment in her case, which is expected within a few hours.
The young Mary arrives from France to Scotland, as the lawful queen of Scots, only to encounter a strong opposition.
Meanwhile, Queen Elizabeth I, angered that with a lawful Catholic queen taking over Scotland will limit her influence there and threaten her right to the throne in England, sends to Scotland her confidant Henry Darnley, who is both an English peer and a Scottish lord, to spy for her and to raise the population against Mary Stuart.
A troupe of itinerant actors stages a play hinting broadly that Riccio is the queen's lover and might be the father of her child.
On the advice of Lord Bothwell, now her lover, Mary has her ill husband brought to Edinburgh, where he dies in an explosion at his home.
He turns away from her, but the treacherous Jacob Stuart still sentences him to death by dragging (in actual history he fled to Denmark where the king treated him cruelly and where he eventually died in prison; all of which is not mentioned in the film).
Mary makes peace with dying and pledges her undying devotion to her ladies-in-waiting, the people of Scotland, and the many men she loved and lost.
The next morning Mary Stuart, in a stunning bejeweled red gown, is led to the scaffold and kneels down in prayer as she awaits the sharp hatchet to fall.
Nevertheless, it is noted that their common son, the future King James I would be a good ruler but, as one of the courtiers says to Queen Mary, he will "have a big heart".
The premiere took place on 1 November 1940 in the München Ufa-Palace ("Luitpold-Theater") The film is punctuated by numerous songs, mostly sung by Zarah Leander: The text was written by Harald Braun, the music by Theo Mackeben.
Das Herz der Königin became a failure in its time and today counts as one of the weakest of Zarah Leander's films.