Tyrannical King Louis XIV (Louis Hayward) learns that he has an identical twin brother, Philippe (also Hayward), who was raised from birth by his late father's trusted friend D'Artagnan (Warren William) and his faithful musketeers, Porthos, Athos and Aramis. After Philippe falls for the king's betrothed, Spanish Princess Maria Theresa (Joan Bennett), 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.