 A Time Lord is able to conceal their Time Lord nature, and become a human, by using the Chameleon Arch – a device that stores their "essence" and memories in an innocuous device such as a fob watch, and replaces them with false counterparts until the object is later re-opened.The process allows them to disguise themselves as humans physiologically and psychologically, meaning they only have one heart and are stripped of non-human powers, and of any memory of having been a Time Lord.This story element was notably featured in Series 3; the Doctor uses it to hide himself from the Family of Blood and becomes a schoolteacher in Edwardian England.<ref name="Human Nature"></ref><ref></ref> His nemesis the Master used it to disguise himself as a human to escape the Time War.<ref name="Utopia"></ref> The story element is later revisited in the Series 12 episode Fugitive of the Judoon when a mysterious, unknown incarnation of the Doctor (played by Jo Martin), is revealed to have been hiding on Earth as a tour guide named “Ruth Clayton” using a Chameleon Arch.