There's an imposter amongst you and your crew.

The Thing is an extremely hostile hive-minded shape-shifting extraterrestrial multi-organism. The Thing has the ability to assimilate other life forms in order to survive and spread. The original physical characteristics of The Thing and its motivations are unknown as it could have assimilated hundreds if not thousands of other species before it crashed on Earth.

The only thing that can defeat it is with a flamethrower to burn it, and sometimes electricity.