Kellman

Professor Kellman was an exographer sent to Nerva Beacon to complete a survey on Jupiter and its new moon, Neo Phobos.

The Voga account
Kellman visited Neo Phobos and discovered it was Voga, the "planet of gold", where he was paid in gold by a Vogan faction led by Vorus to ally himself with a group of CyberNomads and lure them to Voga — as he learnt the asteroid was called — so they could be destroyed by a rocket.

After having done this, Kellman obeyed the Cybermen's instructions to use the Cybermats to infect the Beacon's crew with a plague, which killed off everybody except Commander Stevenson and Lester. The Fourth Doctor later exposed him as the perpetrator and he was taken prisoner. When the Cybermen boarded the ship, he was allowed to transmat to Voga where he tried to find Vorus.

However, civil war had broken out and he was taken prisoner by a faction opposed to Vorus. Kellman explained to Chief Councillor Tyrum what Vorus' plan entailed and went with Harry Sullivan to try to stop the Cybermen from planting their bombs. As he was climbing, Harry accidentally caused a rock fall that killed Kellman. (TV: Revenge of the Cybermen)

The asteroid account
According to another version of events, Kellman discovered a tribe of lost colonists inhabiting an asteroid rich in gold. Without the knowledge of the Nerva crew, he began meeting with the tribe and manipulated them in order to acquire their gold for himself, pertaining to provide them with security. The Doctor later discovered the tribe and revealed Kellman had murdered the son of Coetzee. Kellman tried to flee as Coetzee attacked him with some of his own explosive charges, and he was killed in the resulting rock fall, but not before he managed to fatally injure Coetzee. (AUDIO: Return of the Cybermen)

Behind the scenes

 * Return of the Cybermen, which was adapted from the original version of the scripts for Revenge of the Cybermen, consistently portrays Kellman as more self-centred and villainous than his televised counterpart is ultimately revealed to be. John Dorney, who adapted Return for audio, originally attempted to justify the contradictions in the unused ending by having the fallout of Genesis of the Daleks and the Last Great Time War negate the Return timeline and replace it with the Revenge timeline.