PentoCyleinicMethyldrane (or PCM) was an anxiety inducing drug used by the Company to intensify the submissiveness of its subjects on Pluto.
Around Megropolis One there were eight vapour towers in which PCM was stored, controlled from a central point at Main Control. PCM had a critical temperature of 205 centigrade, maintained at this central control. Prior to his arrest (for stealing and using tablets for "official use" only that counteracted the effects of PCM for the Gatherer and his staff) Bisham's job was in the manufacture of PCM.
PCM was extremely important to the Company as it kept the subjects in order and submissive under their harsh treatment. Bisham noted that the PCM was the source of the Company's power.
For this reason, the Fourth Doctor chose the PCM control as the target of his rebellion. The rebel leader, Mandrel, had previously worked at Main Control, so he knew how to operate the system.
The rebels attacked Main Control and forced the two guards to surrender. Mandrel closed down the vapour towers and maintained the PCM temperature at 70 centigrade. The PCM in the atmosphere fell dramatically to three parts in a thousand. (TV: The Sun Makers)