The conspirators were led by Roberto di Ridolfo, who, posing as an international banker, was able to travel between Brussels, Rome and Madrid without attracting too much suspicion. Ridolfi had discussed his plans with the Duke of Alva in the Netherlands. The plan was to foment a rebellion of the northern English nobility, many of whom were believed still to be Catholic, and marry Mary to Thomas Howard, 4th Duke of Norfolk, the leading Catholic nobleman.
The Spanish were at first doubtful as to the value of the plan; there were obstacles; Philip II of Spain disliked the idea of assassinating Elizabeth; a stable England was needed as a counterweight to France; there was no guarantee that the English population or its nobility were as Catholic in sentiment as the success of the plot demanded.
However, the activities of Sir John Hawkins and the detention in England of Spanish ships carrying large sums of money destined for their armies in the Netherlands caused a worsening of relations between England and Spain, and the Spanish, encouraged by petitions from English Catholics for deliverance, went ahead. But Charles Baillie, a Scot favourable to Mary's party, was arrested at Dover carrying compromising letters, and revealed the existence of the plot under torture. The Duke of Norfolk was discovered to have been funding Mary's party in Scotland and was arrested on September 7 1571. Norfolk was put on trial for treason early in 1572, and executed in June.
Although unsuccessful, the plot concentrated the minds of the English government on assassination attempts on Elizabeth, and, more importantly, exacerbated the Puritan demand that Mary be executed in order to safeguard the English church and state. After the Babington plot failed, Elizabeth had Mary thrown into prison and later executed.