In what has become known as Gerard's conspiracy, a group of Royalists conspired to assassinate England's Lord Protector Oliver Cromwell in May 1654. It was discovered before an assassination attempt could be made and the Protectorate arrested about forty men. Three were tried for high treason. A ring-leader, John Gerard, was found guilty and executed, as was Peter Vowell. Summerset Fox pleaded guilty and was sentenced to transportation. The view of the Royalists and their sympathisers was that the conspirators had fallen into a trap set by Cromwell.
Early in 1654 John Gerard went over to France, where he was presented to Charles II by his cousin, Charles Gerard, Lord Brandon. Soon after his return to England in May 1654, a scheme to assassinate the Lord Protector Oliver Cromwell on his way to Hampton Court was discovered. About forty people, among whom were Aubrey, Earl of Oxford, the two Ashburnhams (John and William), Sir Richard Willis, John Gerard and his elder brother Sir Gilbert, were arrested.
It was alleged that in company with a Royalist major, one Henshaw (whom Gerard had met in France), Gerard with 30 other mounted men were to attack the Lord Protector, as he rode to Hampton Court, and, after overwhelming his bodyguard and killing him, to besiege Whitehall, seize the Tower of London, and proclaim Charles II king.