Parts: 4
Writer: Robert Holmes (story by John Lucarotti)
Airdates: 25 January 1975 - 15 February 1975
Production Code: 4C

Doctor: fourth
Companions: Sarah, Harry

Creature Feature: Wirrn

Story: Although attempting to take Harry to the moon in order to convince him that the TARDIS really does travel in space, the Doctor naturally misses his destination. Instead, they materialize on board a defunct orbital space station. Soon it becomes evident that the station is actually a repository for all of human knowledge - and even quite a few humans. The Ark is the hope of humanity, far in Earth's future when the planet has been destroyed by solar flares.

But the time for the sleepers to awaken has long past - the cables to their "alarm clock" have been cut through. Harry learns why when he opens a closet door and is surprised by a huge dead insect falling out onto him. With the help of Vira, the first of the humans to awaken, the Doctor learns that the insect is a Wirrn Queen...and its egg sac is empty.

As a new generation of Wirrn begin growing in the solar stacks, and with the humans' leader Noah slowly turning into one of the insects himself, the future of humanity looks bleak indeed. The Doctor, Sarah and Harry must help Vira save the sleepers from being parasitized by the formidable Wirrn, or else they will die along with Earth's greatest minds.