The Gunfighters

From http://www.bbc.co.uk/doctorwho/classic/episodeguide:

The TARDIS arrives in the town of Tombstone in the Wild West and the Doctor, having hurt a tooth on one of Cyril's sweets, decides he must visit a dentist. The local dentist is Doc Holliday, currently engaged in a feud with the Clanton family. Lawmen Wyatt Earp and Bat Masterson are meanwhile doing their best to keep the peace.

The Doctor, Steven and Dodo narrowly survive a lynch mob, the attentions of Holliday and Earp and various other dangers; they finally return to the TARDIS after witnessing the famous gunfight at the OK Corral, in which the young Clanton brothers and their gunman ally Johnny Ringo are all killed by Holliday, Earp and Earp's brother Virgil.

3 out of 91 (3.3%) raters say this story requires a previous story.