Zeta Minor was the planet farthest out in the known universe. It contained a link to a dimension of anti-matter.
Zeta Minor had a jungle-like terrain and was home to the anti-matter creature. Professor Sorenson believed that the planet was alive and, as he told Baldwin, that it "[watched] every move they [made]." The surface was divided into sectors for the purpose of designation.
Salamar described its location as being "beyond Cygnus A", "as distant again from the Ortoro Galaxy as that galaxy [was] from the Anterades. [It was] on the very edge of the known universe." (Planet of Evil)
In the year 37166, Professor Sorenson led an eight-man expion to Zeta Minor in search of an alternate power source that would save the Morestran civilisation.
When they found the first high quality strain, the expion began to suffer from casualties — Lorenzo was the first — and the strain was found to have vanished. Sorenson believed the planet had "taken it back".
While in sector 5, Sorenson and Baldwin came across a vein with over seventy percent purity. The professor was unwilling to leave, despite the approach of nightfall, lest the vein should vanish again. At the expion base, Braun was killed by the anti-matter creature. Baldwin returned to the base alone and, finding himself under attack, pressed the automatic distress button before he too was killed.
The Fourth Doctor and Sarah Jane Smith came to Zeta Minor in response to this signal, landing shortly before a Morestran probe ship began orbiting the planet. This ship was captained by Controller Salamar, who rejected Vishinsky’s request for a low level scan of the planet in order to conserve fuel and ordered him to take a party down to the surface.
Sarah soon found the planet to be having a strange effect upon and felt as if she "was being drawn from her body". Finding the dessicated corpse of Braun, the product of the anti-matter creature, the Doctor thought they must have landed months too late.
they escaped and discovered anti-matter in a pool-like state on the planet’s surface. O’Hara became the next victim of the creature and Ponti fell into the "pool".
By helping the Morestrans fend off an attack on their probe by the creature, they gained their confidence. The presence of anti-matter minerals on the probe prevented it from taking off as the creature desired their return. The Doctor took a small contained of these minerals before heading through the jungle to negotiate with the creature. He plunged into the "pool" where he promised that the anti-matter would be restored to this other dimension. The minerals he kept about his person allowed him to re-emerge.
The majority of the minerals were removed from the ship, but Sorenson secreted some. The ship took off, but was unable to escape the planet’s gravity due to this concealment. Under attack from Sorenson, whom the minerals had infected, several crew members were killed. The Doctor used his TARDIS to return to the planet’s surface with Sorenson and the remaining minerals – both of which were submerged in the anti-matter "pool". The promise fulfilled, the creature restored Sorenson to his former self and allowed the ship to leave. (Planet of Evil)