S04E21: Zero Sum

After a swarm of bees mysteriously kill a postal worker in an enclosed restroom, Assistant Director Skinner covers it up by destroying the evidence. In order to complete the task he poses as Agent Mulder, however his deception backfires. When Mulder finds out and confronts him, Skinner reveals that he is working for the Cigarette-Smoking Man in order to obtain a possible cure for Scully's cancer.