After the failed Northern Rebellion of 1569, Thomas Howard, 4th Duke of Norfolk was kept a virtual prisoner, under house arrest at Howard House. He retired from public life and many of his roles and responsibilities as the leading peer in England were deputised to others.

Early in 1571 Mary began to seek a solution to her problems outside England. Roberto Ridolfi, an energetic and imaginative Florentine banker, who had already been in Walsingham's custody for his implication in the events of 1569, acted as papal agent and intermediary between Mary, her agent, the bishop of Ross, her English supporters, and Spain. An extensive, overmanned, and vulnerable conspiratorial network, including the servants of the principal participants, planned the release of the Scottish queen, her marriage to the duke, and, with Spanish military assistance, Elizabeth's removal in favour of Mary and the restoration of Catholicism in England. The success of the plan required Norfolk's approval and involvement. An initial approach by the bishop of Ross, forwarding ciphered letters from Mary, failed to secure his support. However, Norfolk reluctantly agreed to meet Ridolfi, as a result of which he gave verbal approval to the request for Spanish military assistance. His name was also subscribed to letters to the pope, Philip II, and the duke of Alva, Philip's commander in the Netherlands.

The Ridolfi plot was exposed more as a result of a series of accidents than of deliberate official investigation. Servants of Mary's agent, the bishop of Ross, and of Norfolk did not prove adept at avoiding suspicion and detection as they travelled with ciphered letters and money for their supporters. On about 12 April 1571 Charles Bailly, Ross's servant, was caught with letters for him at Dover. Then, on 29 August, Norfolk's secretaries William Barker and Robert Higford entrusted to Thomas Browne, a Shrewsbury draper, what purported to be a bag of silver coin for delivery to Laurence Bannister, one of the duke's officials in the north of England. Browne grew suspicious of the bag's weight, opened it, and discovered £600 in gold from the French ambassador and destined for Scotland on Mary's behalf. It also contained ciphered letters. Because he knew that Norfolk was under suspicion Browne reported his find to Cecil, now Lord Burghley. Higford and Barker were examined, the letters were partly deciphered, and a search for the cipher key at Howard House uncovered a ciphered letter from Mary Stuart hidden under a doormat. From this point the duke's failure to honour his submission was revealed and his complicity in a wider treasonable conspiracy was established.

Norfolk's servants were arrested and interrogated and confessions were extracted from them by threats or application of torture. Sir Thomas Smith and Thomas Wilson were sent to confront Norfolk, who claimed that the £600 was for his own private purposes. The deciphered letter, however, proved that he was lying. Unaware of his servants' confessions or the survival of letters which, contrary to his instructions, had not been burnt, he denied the charges against him. On 7 September the queen's warrant for conveying him to the Tower arrived. Thereupon the duke admitted a degree of involvement in the transmission of money and correspondence to Mary's Scottish supporters, before he was taken through London to the Tower. A servant of the Spanish ambassador reported that those who witnessed this were vocal in Norfolk's favour. Guerau de Spes, the ambassador, referred to ‘this popularity of his amongst the common people’. He observed that ‘the concourse of people was so large and the shouts so general that a very little more and he would have been liberated’ (CSP Spain, 1509–25, 335).

In the following weeks the duke and his servants were subjected to repeated examinations which, together with his confessions, confirmed his disloyalty to Elizabeth, the breach of his own submission to her, and his complicity in conspiracy. In January 1572 he was tried and convicted on three counts of high treason, and on 2nd June he was beheaded on Tower Hill.

By now it was becoming clear to Elizabeth's supporters and the nobles that formed her inner circle that Mary had become the focus of conspiracy and was seen as the Catholic figurehead in any attempt at rebellion against the crown.