1585 Poley was placed, apparently by Blount, with Sir Philip Sidney who had recently married Sir Francis Walsingham's daughter Frances and, as a part of the marriage settlement, was living in Sir Francis's house in Seething Lane. This meant that Poley (whether it was planned or not) could claim to have regular access to Sir Francis without there being any suspicion that he was actually working for him. On 18 January 1586 Morgan wrote that Poley "is placed with the Lady Sidney, the daughter of Secretary Walsingham, & by that means ordinarily in his house". When Sir Philip left England to fight in the Netherlands in November 1585, Poley remained behind with Frances, and remained there until Sir Philip's death in October 1586, nearly a year later. In January 1586 he was actually asked to organise the delivery of a packet of letters to Mary, Queen of Scots. Apparently fully trusted by the Catholics, Poley nevertheless delivered the details to Walsingham. In March/April that year he was also able to reveal thoughts of plot to kill Leicester to the Earl himself, and wrote back on Leicester's behalf to discover the details. The Babington Plot In June 1586 Poley's cover as a Catholic sympathiser in Sir Francis's household was still working as he was asked to help obtain passports from Walsingham for Sir Anthony Babington and his fellow-conspirator Thomas Salisbury. From then on Poley became Babington's "servant and companion" and was often seen in his company despite doubts about Poley's trustworthiness being expressed by some Catholics. In the first two weeks of August, the conspiracy came to its fateful conclusion. On 2 June the conspirators had met and dined "in Poley's garden", the house having actually been requisitioned by the government from a Queen's Messenger, Anthony Hall. Babington found Poley copying some of his papers, but Poley was able to talk his way out of it. On the following day, Poley briefed Sir Francis and on 4 June passed messages from him back to Thomas Walsingham, who seems to have been the case officer. Although the conspirators fled they were all caught. In his cover as one of them Poley was committed to the Tower on 18 August, "charged to have dealt treacherously", but the Catholics weren't fooled. Anthony Babington, who was executed on 20 September, feared that it was Poley who had betrayed him, but he could never bring himself