Main Control was a facility in Megropolis One on Pluto. It was used to control various functions within the city, notably: water, lighting and the cities eight vapour towers. It was this last feature that had the Fourth Doctor lead his rebel force into attacking Main Control. Prior to becoming an outlaw, Mandrel had worked at Main Control and so he knew that only two work units would be on duty at any one time.
In order to prevent Leela becoming a victim of the steaming, K9 entered a vent at Main Control. From there he was able to stop the water output, halting the execution. The Doctor used this delay to rescue her.
K9 operated as a guard for Main Control along with Cordo, Bisham and Mandrel. They watched on the room's television as the public video system broadcast a message in favour of the rebels, after the Doctor took over the controls. After the fighting spread across the city, Bisham, Cordo and Mandrel left the original guards in control, albeit on the rebel side. (TV: The Sun Makers)