The first couple seasons are dead serious. The episodes are bleak, scary, dark, and the only humor is contained in a line here or there. Entire episodes never even have a straight undertone of humor throughout the episode.
Written by flukeman actor, Darin Morgan, brother of Glen Morgan, "Humbug" is the first truly irreverent episode of the series, and certainly not the last. As the show progresses, more and more episodes are slanted in the funny direction, a product of the writers finding comfort with the characters and producers willing to take chances.
Mulder and Scully visit a sideshow town where a brutal murder has occurred. They meet an array of characters--characters in the purest sense. These are performers, each with special abilities or traits. However, some refuse to perform, instead making a living as a normal person would, running a motel, etc. This creates a great atmosphere for the episode and the script is as sharp as it gets, blending both the indignation of some of the residents and the cold reality that they are just different.
Mulder and Scully approach the case, looking at one of the "freaks," but Mulder points out that they may be discriminating by starting with this attitude. In the end, the Fiji Mermaid is none other than the twin fetus of Lanny, who is suggested to have been eaten by Conundrum. And there are still plenty of scary horror stuff alongside the overly funny scenes, making it a very well-rounded episode.