D84 worked alongside another agent, 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 at the cost of his own life, deactivating a killer robot and allowing the Doctor enough time for his plan to work. (TV: The Robots of Death)