The conspirators placed 36 barrels of gunpowder beneath the House of Lords. Guy Fawkes was found guarding them during a search of the undercroft beneath Parliament on the night of 4 November 1605.
The gunpowder was intended to detonate during the State Opening of Parliament the following day. The blast was meant to kill King James I, members of both Houses and leading officials, creating an opportunity for the conspirators to launch a wider Catholic uprising.
The powder was stored in wooden barrels in an undercroft rented by Thomas Percy. Fawkes, an experienced soldier who had fought in the Spanish Netherlands, was given responsibility for igniting it. The warning letter sent to William Parker, Lord Monteagle, helped prompt the search that exposed the plot.
Some accounts describe the powder as “decayed”, but that did not necessarily mean it was harmless. Modern reconstructions have suggested that the quantity—about one ton in some estimates—could have devastated the chamber and caused many deaths. The plotters never got the chance to test it.