Sorenson discovered rich veins of anti-matter minerals on the planet with which he could refuel the Morestran sun. His monomania concerning this scientific pursuit, coupled with the deaths of all seven of his fellow expedition members, left him with some mental derangement, which was immediately noticeable to Vishinsky's landing party.
This party came from a Morestran probe ship, captained by Controller Salamar, that had come in search of Sorenson's expedition. He brought his mineral samples on board the ship. When the Fourth Doctor reasoned that the samples would have to be left behind if the ship was to ever leave, Sorenson secreted a canister.
The anti-matter infected Sorenson, causing his degeneration into a beastly form with glowing red eyes. He attacked and killed Morelli, De Haan and Reig, leaving behind the same desiccated corpses as the anti-matter creature. By drinking an oral vaccine he was able to revert this change temporarily. The Doctor labelled this creature "anti-man", a hybrid creature whose brain cells had been destroyed causing it to "descend to the level of a brute".
The Doctor confronted Sorenson in his room aboard the ship, confirming that his vaccine was no longer working and had set in motion a series of chemical changes. The professor took the remaining mineral canister and prepared to eject it and his own person into space but transformed again before he could.
Salamar used a neutron accelerator against him and was killed by Sorenson in the attempt. The device triggered the creation of several duplicates of the professor, composed of pure anti-matter. The Doctor shot him and carried him into the TARDIS.
There, having landed on Zeta Minor, Sorenson broke free of his constraints. The Doctor lured him and, in his attempts to strike the Doctor, the professor fell into a 'pool' of anti-matter. The Doctor then returned the last canister and the duplicates disappeared.
The Doctor rediscovered Sorenson by the side of the pool, restored to his former self. He assured him that he was "a very lucky man", released as the Time Lord had kept his promise to return the anti-matter.
Suffering from memory loss, remembering only that he had discovered a promising source of energy, he was convinced by the Doctor that he had abandoned that line of thought and had been looking into the kinetic force of planetary movement as an alternative. (TV: Planet of Evil)