The 'shapeshifter' plot is one of the most frequently occuring stories in SF. I'm not talking about just the idea of a character that can change their form, like Deep Space Nine's Odo or Marvel's Mystique. There is a fairly specific plot involving a malevolent shapeshifter that has been repeated and reused many times over the decades.
It goes like this:
Somewhere remote and lonely. A space station, or a polar base, or a small research station on a distant planet. The environment outside must be hostile. The protagonists of the story must be trapped in a relatively small area, unable to escape or quickly summon help.
-
A handful of people that inhabit or work in the chosen location. The reader must be able to identify with them, so they are just ordinary people. They certainly aren't elite space warriors or anything like that.
-
The monster. The monster is a shapeshifter. It can effortlessly change form to perfectly resemble any object or being. It wants to kill the people. Usually, it has no obvious motivation. Since it is capable of imitating humans so well, it must, by implication, have some kind of psychic ability in order to pick up the language and so on. This is rarely discussed.
The Plot
Act 1
The people somehow, unwittingly, allow the monster into their habitat. Perhaps it is brought in as a scientific sample ("Look at this interesting egg I found!") or perhaps it's already up to its tricks and it impersonates a human to get in.
Act 2
The monster starts to pick people off one at a time. It may use its shapeshifting abilities to trick the people into killing themselves. (If it were a fearsome fighting machine, there wouldn't be much point in it being a shapeshifter). It may, for example, lure people into airlocks - various things can happen at this point, it's not really important what, other than that most of the people are killed.
Act 3
The remaining people somehow figure out what's going on. There are usually only two people left by this point. They become very scared. They attempt to either escape (perhaps in a cramped escape pod), or to barricade themselves into one small safe area until rescue arrives.
Surprise! One of the two remaining people is, in fact, the monster. They kill the other person.
Obviously, the story doesn't take this exact form every time. The 'suprise' ending (which really isn't a surprise once you've seen/read just one of these stories, unfortunately) tends to be the common theme. It's possible for the monster to be defeated, but that tends to take the sting out a little. A compromise is to have the last remaining person kill the monster at the expense of their own life, by, for example, blowing up the space station. Alternatively, the author may go the other way and hint at yet more carnage to come - if the monster is the only inhabitant of a craft headed for earth, for example, we can assume there will be more killings once it arrives.
Examples
I'm certain there are plenty more, these are just the ones I happen to remember. Also, I wonder if there are some older examples - I wouldn't be surprised if you could find stories about changelings a hundred years ago. If anyone could /msg me with further additions that would be excellent.
Finally, here's my take on the genre. It's written in the style of the old SF short stories I grew up reading. So, I've jumped right into the middle of the story, as is conventional, and, naturally, the cast are all men that call each other almost exclusively by their surnames.