Parts: 2
Writers: Bob Baker, Dave Martin
Airdates: 22 February 1975 - 1 March 1975
Production Code: 4B

Doctor: fourth
Companions: Sarah, Harry

Creature Feature: Sontarans

Story: The transmat from the Ark brings the Doctor, Sarah and Harry to the surface of Earth, far in their own future. Sarah is shocked to see how much Piccadilly Circus has changed - far from being an area bustling with people and traffic, it has turned into a barren scrubland. While the Doctor works on fixing the transmat circle, Sarah and Harry go off exploring.

Harry accidentally falls into a ravine that is too steep for him to escape. Upon examining the ground where he fell, Sarah discovers that the edge was covered up with branches...implying that the Earth is not as barren as it seems. Indeed, the crew of a Galsec ship is on the planet. One of their number, chased by a robot, falls over a cliff and dies. When the Doctor investigates, he is found leaning over the body by the remainder of the crew and taken prisoner.

The TARDIS crew learns that the members of the Galsec ship are being slowly destroyed by an alien hiding in the rocks. Its robot pursues and captures them, then takes them back for experimentation and torture. Before long, Sarah suffers the same fate, and she is horrified to see that the alien in the rocks is none other than a Sontaran. Unless the TARDIS crew can stop his horrible experiments, a Sontaran battle fleet will invade the entire galaxy...