The Man in the Iron Mask (1939)

Tyrannical King Louis XIV learns that he has an identical twin brother, Philippe, who was raised from birth by his late father's trusted friend D'Artagnan and his faithful musketeers, Porthos, Athos and Aramis. After Philippe falls for the king's betrothed, Spanish Princess Maria Theresa, Louis imprisons him, forcing his brother to don an iron mask that will slowly suffocate him -- and it's up to D'Artagnan to rescue him.