The robots were bipedal, humanoid-shaped, metallic, identical, with hands consisting of a thumb and two jointed fingers. Each was outfitted in a tan EVA spacesuit with a rocket pack. They were able to manipulate power tools. They were programmed to perform specific repair and maintenance functions in unison on the exterior of the nacelles, secondary hull and primary hull of a starship. The were designed at a research center on Venus.
While on the edge of sector H-100, the USS Enterprise conducted an experimental program with a metal mob of at least 17 members. Provided by Earth, the robots conducted maintenance repairs on the ship over a week-long period during the day shift, and were secured in Machinery Bay 2 during the night shift. Spock programmed their specific maintenance tasks. The robots performed admirably by all accounts. However, two of them were taken over by disembodied consciousnesses from Varka, a planet within the sector. One robot forged a message to Starfleet Command using the secret authorization code of James T. Kirk that the Enterprise had been compromised and needed to be destroyed, prompting a military assault by the Second Fleet. A second robot overrode airlock seals in an attempt to suffocate the crew, but was shot down by Kirk with a phaser rifle. The final disposition of the metal mob was unknown. (TOScomic: "Thorpex")