The sixteenth season of British science fiction television series Doctor Who, known collectively as The Key to Time, began on 2 September 1978 with The Ribos Operation and ended with The Armageddon Factor.

[1] The name refers to a powerful artefact, the segments of which are what the Fourth Doctor and his companions, Romana and K9, search for during the season.

Afterward, the Doctor decides to install a device called a randomiser into the TARDIS' navigation system for a period of time to make his following voyages unpredictable to evade the Black Guardian.

Tom Baker continued his role as The Fourth Doctor, and saw the introduction of Romana played by Mary Tamm.

All releases are for DVD The Ribos OperationThe Pirate PlanetThe Stones of BloodThe Androids of TaraThe Power of KrollThe Armageddon Factor