Sorry this is a bit late. 11:30 AM is probably not a good time to decide I hate the post I have scheduled for noon and start writing a replacement…
It has been observed before that Americans have a particular obsession with the concept of the interloper, the seemingly innocuous neighbor who is actually a terrifying Other in disguise. Certainly this fear is not unique to American culture, but the U.S. does seem particularly prone to panics over it, from witchcraft scares in the Colonial period, to the fear of “seditionists” in the early 19th century, anti-immigrant panics in the later 19th century, the Red Scare, the Yellow Menace, communists in the State department, Satanic Panic, pedophiles in your neighborhood, the gay agenda, President is a secret Muslim, terrorists are plotting to blow up your hick-town suburban mall… There is an undercurrent of paranoia that appears a permanent fixture of American culture, just waiting to burst out at its current target.
And of course it’s in our fiction too. The science fiction classic The Puppet Masters posits people who look completely normal (while clothed, anyway), but are actually under the control of evil alien parasites; given its publication in the 1950s, it’s usually interpreted as a sort of allegory for communist infiltration and the fear thereof, but really it can be adapted to any of the U.S.’s paranoid panics, which is why its basic concept (and the closely related “pod people” from Invasion of the Body Snatchers) keep getting repeated.
Star Trek is no exception. Mind-control parasites were introduced in the first season of Star Trek: The Next Generation as a hasty replacement for a rejected plotline involving an attempted military coup on Earth. Because of course We (or future-space-We in this case) would never do such a thing, except as a result of infiltration by Them. You know–Them, the canny, evil ones who are always trying to infiltrate, undermine, and destroy Us. They hate our freedoms, you know.
The plan was for the parasites (called “bluegills”) to be forerunners of the new series villain, the Borg, but a writer’s strike, major staff turnover, and budget issues got in the way. When the Borg finally did show up, any relation to the bluegills was gone, and with them the metaphor–the Borg have presented many faces over the years (massive corporate behemoth, dark mirror of the Federation, and the most enduring and least interesting, zombies), but devious, sneaky interlopers has never been one of them.
But we definitely got more of those! Deep Space Nine introduced the Changelings, shapeshifters who could replace people with dopplegangers to act as agents in their war of aggression against the Federation. But cleverly, it turned the classic infiltration narrative inside-out in the two-parter “Home Front”/”Paradise Lost”–instead of secretly replacing key figures and subverting the Federation government, the Changelings deliberately allowed the presence of a Changeling (who later claims there are four, but there is only evidence for one) on Earth to be discovered, and then relied on the paranoia and suspicions of the legitimate military leadership do their work for them.
Oddly, the Changelings barely show up in Star Trek Online, and there is no case I’m aware of where they engage in this kind of doppleganger trick. I say oddly because STO is borderline obsessed with the infiltration narrative. The opening premise of the game, war between the Federation and Klingons, is the result of a different species of shapeshifters, the Undine (renamed from Species 8472, which is what Voyager called them) replacing key figures of various governments. This in turn is revealed to be the result of someone else invading Undine space with fake Klingon and Starfleet ships. Over the course of one story arc based on Deep Space Nine, the player has to deal with a ship taken over by Undine infiltrators, then Deep Space Nine itself taken over by Undine infiltrators, and then a few missions later Deep Space Nine gets taken over again by dopplegangers, this time its crew’s evil Mirror Universe counterparts.
Later in the game, the bluegills make a comeback, controlling the leadership of another species that has suddenly become very aggressive and destabilized the region (they are, of course, working for the same people that sent the fake ships after the Undine). Meanwhile, the Romulans have mind-controlled spies (and are being manipulated by–guess who–the people that sent the fake ships), the Undine pull the same infiltration trick on Deep Space Nine, but this time on Earth Spacedock and nearly destroy the place…
And this is a real problem. Part of the aesthetic of Star Trek is that the unknown is an opportunity–a source of excitement at the possibility of discovery. The insular, paranoid fear of the Other that underlies the infiltrator narrative works directly against that.