Guy was introduced to Robert Catesby and Thomas Percy who,with others, were plotting to kill the king and many members of parliament. They wanted to blow up the houses of Parliament at the opening session and so gain power in English again. However, one of the plotters wrote to his brother-in-law , warning him to stay away from parliament on 5 November 1605 because of "an unseen blow". This letter was takan to Robert Cecil , head of the government secret service.A search of the cellars found Guy with his gunpowder ready. He was tortured into confessing details of the plot and on 27 January 1606 was sentenced to be hung, drawn and quartered the punishment for treason. 5 November was declared a day of thanksgiving for deliverance.