En route to London, the TARDIS picks up a faint distress call from the edge of the known universe and the Doctor promptly dashes to the rescue.

The TARDIS arrives on the planet Zeta Minor in the Morestran year 37,166, where a Morestran scientific expedition has been slowly killed off by forces unknown. Now only Professor Sorenson is left, and he is not all that he seems to be.

The TARDIS's arrival coincides with that of a military ship, the Probe, from Morestra, sent to find out exactly what has happened to their expedition. Sorenson is confused and unable to supply the answer, but when members of the military crew start dying, the logical suspects are the Doctor and Sarah, strangers who just happened to arrive as the killings started.

The real cause is in fact a creature comprised of anti-matter, which emerges onto the planet to try and retrieve the anti-matter that Sorenson has taken from the crossing-point of the two universes, a bottomless black pool deep in the forest.

The creature will not allow the Morestrans to leave with anti-matter on board their ship and when they try, the forces that hold it intensify until they are held stationary in space, struggling against the increasing pull back to the planet. 

The Doctor realises that Sorenson himself is infected with anti-matter and is becoming anti-man, a bestial creature capable of draining the life from other beings. The ship's commander, Controller Salamar, who has become increasingly unstable under the great pressures of command, cracks, and goes after Sorenson with a neutron accelerator. Instead of killing him, the radiation from this causes the creature which Sorenson has become to multiply. The ship is soon overrun with snarling deadly creatures, able to walk through doors and intent on sucking the life from the crew.

The Doctor finds the original Sorenson anti-man and takes him back to the planet in the TARDIS, pushing him into the pit. This fulfils an earlier bargain the Doctor had made when he too fell into the pit. Sorenson is returned intact, and the Morestran ship is released.

The Doctor takes Sorenson back to the Morestran ship and then he and Sarah leave to try and keep their appointment with the Brigadier in London.

_{Synopsis from Doctor Who: The Fourth Doctor Handbook by David J. Howe, Mark Stammers and Stephen James Walker, reprinted with permission; further reproduction is not permitted. Available from Telos}