Parts: 4
Writer: Lewis Marks
Airdates: 27 September 1975 - 18 October 1975
Production Code: 4H

Doctor: fourth
Companion: Sarah

Story: The Doctor and Sarah respond to a distress call coming from the planet Zeta Minor, where a Morestran expedition has been collecting and analyzing the yellow crystals found in certain parts of the planet. But the expedition is being slowly killed off by an invisible creature lurking in the jungles of Zeta Minor, and soon only one remains - Professor Sorenson, leader of the expedition.

When another Morestran ship arrives in response to the distress call, they are immediately suspicious of the time travellers. Zeta Minor is on the edge of the known universe, and the likelihood of their "passing through" and hearing the distress call is extremely slim. As more deaths occur, the Doctor and Sarah are further implicated, and their executions seem sure to follow.

But what the Morestrans do not realize is that Zeta Minor is the boundary between the normal universe and the universe of anti-matter. The crystals that Sorenson is attempting to take from the planet are anti-matter crystals, and the creatures that dwell in that universe will not allow their removal. And to make things worse, the anti-matter begins to affect Sorenson in strange ways...and could cause the destruction of them all.