D84

D84 was a unique sandminer robot, completely independent of a Super-Voc and with a high level of logical reasoning.

D84 was an undercover agent disguised as a "Dum" and placed on board Storm Mine 4 as a precaution to threats that Taren Capel would lead the robots into a revolution against humans.

Mission
D84 worked alongside another agent, Ander Poul. D84's cover was first blown by Leela who, on finding a third corpse, saw that D84 was in the room. To assure her he had no part in the murder he had to speak, but Leela promised to keep this a secret. The Fourth Doctor later found out from Leela that the D84 could speak and learned the robot's true purpose on board the sandminer.

The Doctor and D84 searched the ship for evidence that Taren Capel was on board and soon found his secret workshop, where he reprogrammed the robots to kill. Afterward, they found a deactivated robot and used its head for an anti-robot transmitter. D84 was damaged when Taren Capel returned to his workshop and found the Doctor, but as a last act the robot bravely activated the transmitter, sacrificing himself in the process, deactivating a killer robot and allowing the Doctor enough time for his plan to work. (TV: The Robots of Death)

Behind the scenes
The K9 episode Liberation contains an in-joke referencing this character, as one of the corridor doors in Dauntless Prison is labelled "D84".