The whole impostor situation started in the early golden days of MIRA. A distant solar system was discovered, named the JRSC System, and explored extensively for research and materials. One of the planets in the system, JRSC-5, was found to hold alien life.

Among the various flora and fauna of JRSC-5 was a very large species of strange beast. Capable of shapeshifting, not quite mammal yet not quite reptile, not quite anything humanity had ever seen.

Of course, MIRA jumped at the chance to research alien life. Field researchers observed natural behaviors, took samples, caught live specimens. Eventually there were even whole bases built on other planets dedicated to further research on this specific species.

They did not anticipate the intelligence they were capable of.

See, Impostors, as the scientists had taken to calling them for their habit of mimicking human appearances, aren't just mindless creatures. They're a fully sentient species, they just operated differently than humans, spoke in a way humans didn't recognize as language, lived among untouched nature as opposed to buildings or society.

So of course they didn't take too kindly to their friends and family being taken.

Impostors still on the planet began to infiltrate MIRA ships. Impostors in captivity began breaking out and assuming identities of crew members. Some did it just to escape capture, some for survival, some for revenge, some just cuz it was another source of food. Impostors gained their reputation.

This unofficial war was never publicized to those outside of MIRA.

AAAAND that's basically the deal! Pretty unfortunate for everyone involved, especially the innocent Imps just trying to survive and the crewmates just trying to make a living.