Parts: 4
Writer: Christopher H Bidmead
Airdates: 26 January 1984 - 3 February 1984
Production Code: 6N

Doctor: fifth
Companions: Tegan, Turlough

Creature Feature: Tractators

Story: The TARDIS arrives at a point in time beyond which it cannot travel - the boundary of known time. It materializes within the Veruna system, near the planet of Frontios, where the last survivors of the human race fled just before the Earth fell into the sun. Tegan and Turlough wish to visit, but the Doctor refuses - they are not supposed to be there at all, and any interference could destroy the new colony.

But despite his intentions, the TARDIS is pulled down to Frontios during a meteorite storm. The inhabitants are struggling to survive - food is rationed and admittance to the original colony ship and its supplies is severely limited. The meteorite storms, which began twenty years ago after the colonists had lived through thirty peaceful years, now come almost daily, and the colonists believe them to be some kind of attack. And in one of these attacks, the TARDIS is destroyed, leaving only the Doctor's hatstand behind.

What some of the colonists are not telling is that there have been mysterious disappearances over the years - people pulled down into the earth, alive. When Turlough ventures under the surface along with the daughter of the chief scientist, he finds out why - creatures that once viciously attacked his own planet have invaded Frontios. Their number are growing beneath the earth, and their deadly gravitic powers could be the death of the human race.