Contingency Plan A was the name the Company gave to the process of releasing dianene to kill the population of a city. The Company installed a sprinkler system in Megropolis One that would release the poison if a switch in the Collector's palace was activated.
When the Citizens' Revolution saw an uprising against the Company, the Collector planned to use the switch. He himself was an Usurian, who did not breath air, and so would remain unaffected. Leela stopped him by forcibly preventing him access to it. (TV: The Sun Makers)