After a ship landed on Delta III, Fenner and his superior, Thawn, went to to seek the craft. They came across the TARDIS and mistook the Fourth Doctor for the gun runner, Rohm-Dutt. They took him prisoner and returned to the refinery with him. While they were distracted by their machinery, the Doctor escaped.
Fenner thought the Swampies might as well be slaves, caring little for their livelihood. This affected his treatment of Mensch, who was indeed a slave. Neither Fenner not Thawn considered the Swampies people. Nevertheless, when Thawn murdered Dugeen and tried to destroy Kroll and the Swampies, Fenner was not willing to go along with Thawn until he threatened to kill him too.