Which 2007 episode reintroduced the Master?

The 2007 episode that reintroduced the Master is “Utopia.” It is the eleventh episode of Doctor Who’s third revived series, first broadcast on 16 June 2007, and begins a three-part story that continues with “The Sound of Drums” and “Last of the Time Lords.”

In the episode, Professor Yana appears to be helping the last humans escape the dying universe. The Doctor, Martha Jones, and Jack Harkness eventually discover that Yana is actually the Master, whose memories and identity have been suppressed. After being revealed, he steals the TARDIS and regenerates into John Simm’s younger incarnation.

The common mix-up is to choose “The Sound of Drums,” because that is the next episode and features the Master prominently as Harold Saxon. However, “Utopia” is the episode in which his return is revealed. It also marked the character’s first television appearance since the 1996 Doctor Who television movie.