The planet Zeta Minor lies at the very edge of the known Universe. In the depths of its jungles is a cave in which there is a pitch black pool - a gateway to the Universe of Antimatter. This is home to a creature which can straddle the two dimensions. It guards against anyone taking material from the planet, which could lead to cosmic annihilation. Generally invisible, it could appear in outline to the Doctor, Sarah and the Morestran explorers visiting the planet when fired upon by energy weapons.
The creature would kill by sucking the life-force from its victims - leaving desiccated husks.
The Doctor allowed himself to fall into the pool in order to commune with the creature. He promised that the Morestrans would not remove any of the radioactive crystals which they had mined - to fuel their dying sun. The creature allowed him to go free in order to arrange this.
Professor Sorenson became infected by radiation from the crystals. A chemical he devised could hold the transformation at bay only for a short time, and its effect diminished each time he used it. He would transform into a primordial parody of a man, and could kill in the same way as the creature from the pool.
When exposed to a radiation source, the transformation became permanent, and outline splinters were created, which roamed the Morestran ship - killing all they encountered.
The Doctor managed to capture Sorenson and took him to the cave in the TARDIS, where the professor fell into the pool. The creature allowed him to live, freeing him from the contagion, and leaving him with no memory of recent events. All the outline splinters vanished when this happened.
Appearances: Planet of Evil (1975).
- See also "S is for... Sorenson".
- The monster's appearance was inspired by the Monster from the Id, a piece of Disney animation in the classic 1956 sci-fi movie Forbidden Planet, whilst Sorenson's transformation comes from various film & TV versions that have been produced of The Strange Case of Dr Jekyll and Mr Hyde.