Following the death of Elizabeth I in 1603, King James VI of Scotland succeeds to the English throne. Catholic Robert Catesby devises a treasonous plot to assassinate James, but he needs allies.
As the King's network of spies close in on them Catesby and Wintour try to raise an army for their plot across mainland Europe. The plotters then gather in London and a plan is formed.
On the eve of the State Opening of Parliament the plotters load the barrels of gunpowder into the tunnels beneath the Palace of Westminster. Father Garnet is under pressure to reveal the plot.