Which Doctor Who story first revealed the Doctor's species as Time Lord?

The War Games was the Doctor Who story that first revealed the Doctor’s species as the Time Lords. The ten-part Second Doctor serial introduced the Time Lords and their home world, later identified as Gallifrey.

Broadcast in 1969, The War Games follows the Doctor, Jamie, and Zoe as they uncover an alien operation that abducts soldiers from different periods of Earth’s history. The soldiers are brainwashed and forced to fight simulated wars so the alien rulers can create a powerful army.

When the Doctor seeks help, the story introduces the Time Lords as an advanced civilisation capable of judging and controlling time-travelling criminals. They place the Doctor on trial for repeatedly interfering in history and punish him by forcing him to remain on Earth in exile, setting up the Third Doctor’s era.

The story is sometimes confused with The Deadly Assassin or The Invasion of Time, which explore Time Lord society much more deeply. The Three Doctors also features the Time Lords, but it came later, in 1972–73.