Juan de Padilla, one of the main leaders of the Comuneros, is executed in the presence of his wife, Maria Pacheco, who, with his son, decides to avenge his death. Maria, Spanish lady of the highest rank, of delicate health but strong in spirit, heroic reacts and participates in dangerous exploits against the tyranny of King Charles V, becoming "Leona de Castilla", symbol of the oppressed popular liberties.