Under James I, Northumberland was a long-term prisoner in the Tower of London, due to the suspicion that he was complicit in the Gunpowder Plot.

His father died, an apparent suicide, in the Tower of London, where he was being questioned about his allegedly treasonable dealings with Mary, Queen of Scots.

They had four children: Though it did produce a male heir, Algernon, the marriage was not successful, and the couple separated after a time, despite efforts by the queen, who was fond of Dorothy, to reconcile them.

Through Thomas Percy, Henry received loosely worded assurances of religious tolerance from James.

[7] This theory of the "diabolical triplicity" rested on innuendo, about the occult interests supposedly cultivated by the intellectual circles led by Percy and Raleigh, and possibly on the traitorous intent suggested only by rumours from the 1580s that Percy would marry Arbella Stuart, the next heir to the throne after the King and his offspring.

[10] In 1604, the house was visited by the Jesuit Henry Garnet and the Spanish ambassador Juan de Tassis, 1st Count of Villamediana.

However, the Star Chamber did not have sufficient evidence to convict him of misprision and was unable to disprove his claim that he planned to be present at the fatal meeting of Parliament.

[13] He regularly met scholars whom he patronised, including Thomas Harriot, Walter Warner and Robert Hues, who were known as the "Earl of Northumberland's Magi".

Together with Sir Walter Raleigh, who had preceded Northumberland to the Tower with a death sentence hanging over him, they discussed advanced scientific ideas and smoked tobacco.

Frances promoted the marriage of his second daughter Lucy Percy to James Hay, 1st Earl of Carlisle.

[13][16] After his release, experiencing deafness and poor eyesight,[13] he went to the Bath Inn, later referred to as the Arundel House, to regain his health.

He was a patron to Thomas Harriot, Nicholas Hill, Robert Hues, Nathaniel Torporley and Walter Warner.

[17] The astrologer John Dee, nearby Syon House at Mortlake, was also a friend of Henry, and their circles overlapped.

It has been argued that this refers to a circle of scientific investigators who met at Syon House, though other commentators think the word "school" is a misprint for something like "shawl".

Frances Yates comments on this hypothetical group, supposedly including also George Chapman as the author of Shadow of Night, as arguably part of Raleigh's circle, to the effect that they would be "Saturnians" in the sense of her study.