Scotland, Percy wrote urging him to confess his offences and appeal to the queen's mercy. In 1571 he was elected knight of the shire for Northumberland and, on his brother's execution at York in 1572, he assumed by Queen Elizabeth's permission the title of eighth earl of Northumberland, in accordance with the patents of creation. Mary, Queen of Scots, was in confinement at Tutbury, and he opened communication with her agent John Lesley, the bishop of Ross, at Easter 1571, offering help for her escape. Sir Ralph Sadler suspected his intentions, and on 15 November 1571 Percy was arrested while in London and sent to the Tower of London. On 23 February 1572 he wrote, begging the queen to release him. After eighteen months' detention he was brought to trial on a charge of treason. Thereupon he threw himself on the queen's mercy, was fined five thousand marks, and was directed to confine himself to his house at Petworth. On 12 July 1573 he was permitted to come to London, and was soon afterwards set at liberty. On 8 February 1576 he first took his seat in the House of Lords, and was one of the royal commissioners appointed to prorogue parliament in November. In September 1582 he entertained the French agent, M. de Bex, and looked with a friendly eye on Throckmorton's plot to release Queen Mary. With Lord Henry Howard and Francis Throckmorton he was arrested on suspicion of complicity late in the same year, and for a second time was sent to the Tower. He was, however, only detained a few weeks, and no legal proceedings were taken against him. But he was deprived of the governorship of Tynemouth Castle, though he protested. He was still sanguine of compassing the release of Queen Mary. In September 1583 he invited her agent, Charles Paget, and Paget's brother, Lord Paget, to Petworth, and there he discussed the matter fully. The Duc de Guise was to aid the enterprise with French troops, and Northumberland offered advice respecting their landing. William Shelley, who was present at the interview, was arrested and racked next year, and related what took place. Northumberland's aim, he said, was not only to secure Queen Mary's liberty, but to extort from Elizabeth full toleration for the Roman Catholics. In December 1584 Northumberland was sent to the Tower for a third time. He protested his innocence, and courted