The Chameleon Arch is a device that rewrites every cell of an individual to a specific species. The conversion, which causes extreme pain, also provides a set of false memories to match the new persona. The Tenth Doctor uses it in “Human Nature”, enabling him to hide from the Family of Blood in 1913 as the school teacher, John Smith. He retains a small amount of “residual awareness”, resulting in dreams about his life before the change. The Chameleon Arch stores the Doctor’s Time Lord identity in a fob watch that slots into the device as it is operated. In “Utopia”, Martha discovers that The Master used the same process, generating “Professor Yana” as his persona.