After receiving a tip that Hiroshi Sato was manufacturing Equalist weapons in a factory beneath his home, the Metalbending Police Force conducted a raid on the Sato estate, and subsequently discovered the hidden factory.
Once Tenzin, Korra, ChiefBeifong, and some metalbender officers arrived at the factory, they were confronted by Hiroshi Sato, the Lieutenant, and a number of other Equalists. During the confrontation, Hiroshi put his mecha tanks to use for the first time. At this point, Chief Beifong realized that the tip they had received earlier was part of Hiroshi's plan to lure them into his underground factory. After defeating the invading force, Hiroshi ordered that they be taken to Amon. Asami then arrived at the factory and confronted her father, who asked her to join him and the Equalists. Asami appeared willing to take the offer, accepting an electrified glove from her father, but instead used it to shock him and the Lieutenant; this bought Tenzin, Lin, Korra, Mako, and Bolin, who had found their way to the factory a while earlier, and herself enough time to escape. The remaining members of the police force, however, were captured by the Equalists, and brought before Amon to have their bending removed.
The factory is very large and is accessible via a minecart-like device that carries people deep into the mountainside. The factory has retractable walls made out of platinum - a metal so pure that metalbending has no effect on it. There are two large banners bearing Amon's face hanging from a metal beam in the factory.