When a man's decomposed body is found in the sewers of Newark, New Jersey, Mulder is infuriated when Assistant Director Skinner assigns him to investigate, citing it as a routine murder case. But when Scully's autopsy turns up a parasite living inside the body and a sewer worker is attacked and bitten by a strange creature, Mulder discovers that the culprit is the grotesque mutant known as the Flukeman.
Flukeman was somehow spawned from radioactive waste taken from the Chernobyl accident. Flukeman appears to be a humanoid tapeworm (though the term "fluke" properly refers to a flatworm), and inhabits sewers and sewage tanks. The creature bites people with its suckers and injects a tapeworm (possibly an immature version of itself) into the host, which would eventually exit orally, causing the death of the person.
Mulder and a group of waste disposal workers managed to capture Flukeman, but it escaped soon after. He then followed the creature to a sewer, and at the climax of the episode, Mulder apparently kills Flukeman, chopping it in half with a sewer gate as it tried to escape. At the end of the episode, however, we see the remnants of the creature floating in the sewers, still alive.
Flukeman was extremely popular with the fanbase. Although the creature only appeared in "The Host", he/it was given a reference or two in a couple of later episodes (He appears on some tabloids in "Pusher", and in "The Field Where I Died," Scully, when asked if she could make anything different, states that, "Even if I knew for certain I wouldn't change a day ... well, except for that Flukeman thing. I could have done without that just fine."). Flukeman has also appeared on a fantasy PEZ dispenser, a small statue, and a limited edition figurine. The Flukeman traveled aboard a Russian cargo ship from Ukraine before reaching the United States. Upon learning this during the murder investigation, Mulder and Scully theorized that the creature was the result of intense radiation poisoning experienced by an otherwise-normal sized flatworm. While trying to escape captivity, the Flukeman was severed in half by a closing gate. (TXF: "The Host")
In early 1996, an issue of World Weekly Informer was published with a headline on the front page reading, "He's Back!" The article's subheading read, "Flukeman washed up in Martha's Vineyard" and an artist's drawing of the Flukeman was presented above the article.(TXF: "Pusher")