E is for... Elgin

Mark Elgin was a senior executive who worked for Global Chemicals at their new plant at Llanfairfach in South Wales. He grew concerned at some of the decisions being taken by his boss, Dr Stevens, and of reports of strange deaths amongst the miners who were looking after the abandoned coal mine in the area. He was upset by Stevens' apparent refusal to lend cutting equipment to the mine when an accident occurred, which had trapped a miner and a UNIT operative - Jo Grant - in the pit. He grew suspicious of the actions of one of his colleagues - a man named Fell. He followed him to a restricted area of the plant, where he learned that the company was pouring its chemical waste down into the mine workings. Fell was about to unload a waste tank just as Jo and the Doctor were climbing up the pipe. Elgin intervened and saved them. Fell later committed suicide by throwing himself from a window. When Elgin challenged Dr Stevens about what was going on, he was subjected to powerful mind conditioning - the same process which had turned Fell into a mental slave of Stevens' boss - the company's computer.
His ultimate fate remains a mystery.

Played by: Tony Adams. Appearances: The Green Death (1973).

• The reason we don't know what happened to Elgin is because Adams fell ill during production of the later episodes. He is last seen being subjected to BOSS' mind control, then vanishes from the story. His role is taken up by Roy Skelton, playing a new character named Mr James. James is killed by BOSS when Captain Yates breaks his mental conditioning, so this is probably the fate that would have befallen Elgin.
• The tongue-in-cheek DVD extra Global Conspiracy? has Elgin survive the events of The Green Death. Interviewed in 2004, he claims that Dr Stevens is still alive. (He's now the Director General of the BBC).