Ste-Marguerite citadel, France's most isolated island prison, holds a mysterious captive, whose iron-masked face nobody may see on pain of death. Cardinal Mazarin, who rules France in the name of immature king Louis XIV, needs to marry him off to a Spanish infante to seal a peace treaty, but finds him dangerously flirtatious and diagnosed terminally ill. He sends the unstoppable musketeer Charles d'Artagnan and his three buddies to secretly bring the captive, in his mask. The prison governor's daughter is secretly in love with him and arranges an escape just when the musketeers arrive.