Which Doctor Who serial featured the first appearance of the Master?

Which Doctor Who serial featured the first appearance of the Master? Terror of the Autons did. The four-part serial introduced Roger Delgado as the Master, a renegade Time Lord and one of the Doctor's most enduring enemies.

Broadcast in 1971 during Jon Pertwee's third season, Terror of the Autons follows the Master as he arrives on Earth and attempts to conquer it using the Nestene Consciousness and the Autons. The story also introduced the Master's hypnotic powers, deceptive disguises and trademark rivalry with the Doctor.

A common mix-up is The Deadly Assassin, which is famous for expanding the Master's mythology and showing his decayed appearance. The Keeper of Traken and Logopolis also feature later versions of the character, while Utopia introduces the Master to the revived series. None of those stories is his first television appearance.

The Master became the central villain of Doctor Who's eighth season, appearing in every story of that 1971 season. Delgado's portrayal established the character's urbane, manipulative style, setting a template later actors would reinterpret.