In 1666, he was assigned to arrest a band of papist conspirators led by William Rokeby. He attended Rokeby's meeting at the White Hart in disguise as a beggar, then went to meet Sergeant George Mullens. They returned to the pub with the police and attempted to arrest the conspirators. A fight ensued and was only halted when the Fourth Doctor informed all present that London was on fire.
Both sides worked together to fight the fire. When Rokeby saved a Huguenot and her child, Stoneman-Merritt realised Rokeby was a man of honour and conviction. He prevented Mullens from pursuing Rokeby, ordering him instead to focus on fighting the fire.