D84

D84 was a unique sandminer robot, being 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, while investigating the possibility of Taren Capel's threats of a robot revolution becoming a reality. 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. D84 had investigated the crew of the sandminer but the Doctor pointed out that no-one knew what Taren Capel looked like so he could be masquerading as one of them.

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. Afterwards, 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)

Dimensions in Time
D84 was among the lifeforms who were captured by the Rani and Cyrian and incorporated into the Rani's menagerie aboard the Rani. Along with the rest of the menagerie, he was briefly set loose in Albert Square, where the menagerie menaced the Doctors and their companions. (TV: Dimensions in Time)

Other matters

 * The K9 episode Liberation references this character, with one of the corridor doors in Dauntless Prison being labelled "D84".
 * A picture of D84 can be seen in the 50th anniversary trailer.