Like many people in the company, he was under the mental control of B.O.S.S., the computer which secretly ran Global Chemicals. Unlike B.O.S.S.' other slaves, though, Stevens was allowed some degree of free will, and would often argue or disagree with B.O.S.S. Still, the subtle control the computer exerted over him meant that Stevens always did as it commanded him in the end.
The Stevens Process did make more fuel using less oil, but an unfortunate side effect of this was that it make more waste. The waste was highly toxic. When faced with what to do with it, Stevens was instructed by B.O.S.S. to have the gallons upon gallons of disgusting green sludge pumped into the mines of the nearby Llanfairfach Colliery. Here, it spread a horrific disease that came to be known as the "green death" and also mutated ordinary maggots, causing them to grow to gigantic size and attack people. The maggots themselves also spread the green death. Several miners died.
When UNIT became involved, Stevens clashed with the Doctor and Brigadier Lethbridge-Stewart. At B.O.S.S.' urging, he had the mine lift sabotaged, nearly killing Jo Grant and indirectly leading to the death of Bert Pritchard, and he also sent his chauffeur Hinks to steal a maggot egg the Doctor had recovered. However, the egg had already hatched, and the maggot killed Hinks. Stevens used his connections in the British government to get the Prime Minister and the Minister of Ecology to place UNIT under his direct control, and ordered the Brigadier to blow up the mine hoping to hide the toxic waste, maggots and green death forever. However, the maggots burrowed to the surface and threatened to multiply across all of Wales.
Eventually, he found himself assisting B.O.S.S. in a megalomaniacal scheme to take over the world, by connecting B.O.S.S. to other major computer systems throughout the world. However the Doctor used a crystal he'd obtained from the planet Metabelis III to break BOSS' control over Stevens. He told Stevens that if BOSS succeeded, there'd be more death, more pollution and mutations and disease. A remorseful Stevens then turned against B.O.S.S. and, telling the Doctor to escape, overloaded B.O.S.S.' circuits, causing the evil computer to blow up, taking half of the Global Chemicals building with it.
In the tongue-in-cheek mockumentary Global Conspiracy, Stevens was depicted as having survived the explosion. After the incident, he became director of the BBC and later participated in the documentary entitled Global Conspiracy, which helped bring to light the pollution and death that had happened while the company was controlled by B.O.S.S.
- Stevens' survival is depicted in Global Conspiracy.
- In Malcolm Hulke's novelisation of The Green Death, Stevens' first name is given as Thomas.