In 1666, he was assigned to help Sir Richard Stoneman-Merritt arrest a band of papist conspirators led by William Rokeby. They were interrupted by the Great Fire of London, of which they were informed by the Fourth Doctor. Both the conspirators and the police worked together to fight the fire, against Mullens' wishes.
Mullens suspected the Doctor of starting the fire. His attempts to kill the Doctor were stopped by Stoneman-Merritt. At the first opportunity, he attempted to hunt down Rokeby. He followed the Doctor and Rokeby to the Doctor's TARDIS, but the Doctor shut the door in his face and the TARDIS dematerialised. (PROSE: The Republican's Story)