Mission: Aliens

Sergeant, last night we received a signal that an unknown aircraft of extraterrestrial origin had crashed in the taiga. The locals assured us that at the crash site they observed five creatures very similar to humans, only with gray skin and large almond-shaped eyes. When our group arrived there, the flying saucer and the humanoids were no longer there. The locals said that a group of people and a lot of military equipment quickly arrived on the site, they collected every shard of it and fled. Only Professor Stearns, an ex-military man who is serious about the existence of extraterrestrial civilizations, could have done this. If they fall into his hands and he begins to use them for his own selfish purposes, then surely irreparable things will happen on Earth. Your task is to sneak into the base, to eliminate all the guards and release our brothers in mind. Then you'll just have to leave...