Tosh and incoming producer John Wiles were keen to push the boundaries of the programme, and felt that Cotton might be able to deliver a high comedy.
The idea was that this would, again, be a humorous take on the historical story; this time, the target would be the American Wild West (a setting which William Hartnell would later claim to have suggested), and specifically the infamous Gunfight at the O.K.
Eschewing detailed historical research, Cotton opted to hew closer to the version of the Gunfight which had passed into contemporary mythology, with Wyatt Earp as a stolid enforcer of the law and Doc Holliday as a rascally anti-hero.
To Cotton's chagrin, both Wiles and Tosh resigned from Doctor Who at the end of December 1965, to be replaced by producer Innes Lloyd and story editor Gerry Davis.
Lloyd and Davis disliked the historical genre, believing that the viewing audience was more interested in science-fiction stories, and also felt that the comedic bent of The Gunfighters did not fit with their more serious vision of Doctor Who.
The production team's fears about the quality of The Gunfighters appeared to be borne out when the final three episodes equalled or exceeded the series' lowest Audience Appreciation scores to date.
The Gunfighters is told by Doc Holliday and has Johnny Ringo take the contract to kill the Earps in order to afford a copy of the Cambridge Encyclopedia of Classical Biography.