In summer 1605 Garnet met with Robert Catesby, a religious zealot who, unknown to him, planned to kill the Protestant King James I. The existence of Catesby's Gunpowder Plot was revealed to him by Father Oswald Tesimond on 24 July 1605, but as the information was received under the seal of the confessional, he felt that Canon law prevented him from speaking out. Instead, without telling anyone of what Catesby planned, he wrote to his superiors in Rome, urging them to warn English Catholics against the use of force.