Season 12 episodes

No   Code   Title                    First Shown  Eps  Exists
-------------------------------------------------------------
75    4A    Robot                     28 Dec 74    4     all
76    4C    The Ark in Space          25 Jan 75    4     all
77    4B    The Sontaran Experiment   22 Feb 75    2     all
78    4E    Genesis of the Daleks      8 Mar 75    6     all
79    4D    Revenge of the Cybermen   19 Apr 75    4     all

Notes:

The doctor regenerates into his fourth persona, played by Tom Baker. Harry Sullivan joins when Sarah-Jane tricks him into going into the TARDIS in Robot.

Episode Details

Robot

Still unstable from his regeneration, the Doctor is spending much of his time in the UNIT sick bay. Sarah-Jane decides to use the opportunity to investigate a top secret government Think-Tank. She becomes suspicious that all is not as it should be and that a large Robot under the control of the Think-Tank is responsible for a number of break-ins at top secret military bases.

The Ark in Space

The Doctor, Sarah and Harry arrive on a deserted space station in the distant future. They discover that it contains the entire population of Earth in hibernation who have not reawakened when they should have done because of a fault in the computer. In the meantime, the station has been invaded by the Wirrn, a large insect like animal who have other plans for the sleeping humans...

The Sontaran Experiment

Transported back to Earth of the distant future from the Ark, the Doctor Harry, and Sarah discover that they have unwelcome company - a Sontaran warrior. The warrior is involved in deadly experiments to assess the human race as an adversary prior to a full scale invasion of Earth.

Genesis of the Daleks

The Doctor and his companions are transported back in time by the Time Lords who have decided that the Daleks need to be destroyed. The Doctor's mission is to change history so that they have never existed. Soon they are separated and The Doctor and Harry are captured by the evil Elite of the mad scientist Davros, who is working to perfect his Mark 3 travel machine; the Dalek.

Revenge of the Cybermen

Returned to the Space Station by the Time Ring, the Doctor and his companions discover that it is in the grip of a fatal disease which has killed most of the crew. What is the secret of the asteroid it is being used to mark? Which of the crew is the traitor who is in league with the dreaded Cybermen?