The Plant Operations Center is a section of the Springfield Nuclear Power Plant.
Homer uses a deformed fish to gain entry to the room (the security machine mistook to the fish for Mr. Burns). Homer attempts to release a mixture of "spoiled milk and utility-grade chicken" into the air-conditioning system. He is stopped by FBI Agent Annie Crawford, but Lisa manages to trigger the machine anyway. It's all to no avail though as it is revealed the air-condition does not work.