Brunner was a priest-physician aboard the Lung of Heaven. He was an agent of the Lethean rulers. His job was to expose the White God, a sea deity captured by the Parliament of Rats, as a false god.
When the pirates of the Parliament boarded the Lung, Brunner and Nyssa were taken back to the Parliament as food for the god. They were imprisoned in a room with a mirror. Brunner tried to use his silver staff to defeat the entity that came out of the mirror, but failed. He and Nyssa were rescued by the Fifth Doctor and Constantine, and Constantine brought them back safely to the Lung. (PROSE: The Parliament of Rats)