Percy family in the context of Henry de Percy, 1st Baron Percy

Henry V (16 September 1386 – 31 August 1422), also called Henry of Monmouth, was King of England from 1413 until his death in 1422. Despite his relatively short reign, Henry's outstanding military successes in the Hundred Years' War against France made England one of the strongest military powers in Europe. Immortalised in Shakespeare's Henriad plays, Henry is known and celebrated as one of the greatest warrior-kings of medieval England.

Henry of Monmouth, the eldest son of Henry IV, became heir apparent and Prince of Wales after his father seized the throne in 1399. During the reign of his father, the young Prince Henry gained early military experience in Wales during the Glyndŵr rebellion, and by fighting against the powerful Percy family of Northumberland. He played a central part at the Battle of Shrewsbury despite being just sixteen years of age. As he entered adulthood, Henry played an increasingly central role in England's government due to the declining health of his father, but disagreements between Henry and his father led to political conflict between the two. After his father's death in March 1413, Henry ascended to the throne of England and assumed complete control of the country, also reviving the historic English claim to the French throne.

Thomas Percy (c. 1560 – 8 November 1605) was a member of the group of provincial English Catholics who planned the failed Gunpowder Plot of 1605. He was a tall, physically impressive man; little is known of his early life beyond his matriculation in 1579 at the University of Cambridge, and his marriage in 1591 to Martha Wright. In 1596 his second cousin once removed, Henry Percy, 9th Earl of Northumberland, appointed him constable of Alnwick Castle and made him responsible for the Percy family's northern estates. He served the earl in the Low Countries in about 1600–1601, and in the years before 1603 was his intermediary in a series of confidential communications with King James VI of Scotland.

Following James's accession to the English throne in 1603, Percy became disenchanted with the new king, who he supposed had reneged on his promises of toleration for English Catholics. His meeting in June 1603 with Robert Catesby, a religious zealot similarly unimpressed with the new royal Stuart dynasty, led the following year to his joining Catesby's conspiracy to kill the king and his ministers by blowing up the House of Lords with gunpowder. Percy helped fund the group and secured the leases to certain properties in London, one of which was the undercroft directly beneath the House of Lords, in which the gunpowder was finally placed. The conspirators also planned to instigate an uprising in the Midlands and to simultaneously kidnap James's daughter, Elizabeth. Percy was to remain in London and secure the capture of her brother, Henry.

Northumberland House (also known as Suffolk House when owned by the Earls of Suffolk) was a large Jacobean townhouse in London, so-called because it was, for most of its history, the London residence of the Percy family, who were the Earls and later Dukes of Northumberland and one of England's richest and most prominent aristocratic dynasties for many centuries. It stood at the far western end of the Strand from around 1605 until it was demolished in 1874. In its later years it overlooked Trafalgar Square.