 "Utopia" explains the Face of Boe's cryptic message of "you are not alone" (y-a-n-a, the source of the Master's human name in this episode) in the episode "Gridlock".It referred to the Master, another Time Lord.The concept of using a fob watch to disguise a Time Lord as a human was introduced in the two-parter "Human Nature" / "The Family of Blood" earlier this series, when the Doctor hides as a human, John Smith.