Invasion of the Body Snatchers is a unique film of its era because, despite the nods to communism and the soviets, there are no nuclear weapons to be found. Miles Bennell is picked up by police for acting crazy, and tells his story in the form of a flashback. Miles, who is a doctor, had been hearing people talk about having their family and friends somehow replaced by an identical looking impostor. He calls it hysteria until his friend finds one of these impostors in the process of transforming into him. Now a believer, Bennell tries to warn Kauffman, a psychologist. Before Kauffman can see it, the impostor disappears, which leads to Kauffman to think that Bennell is become hysterical too. Bennell and his friend Becky hide out at his apartment as they learn that most people in the town have been replaced and drained of their humanity. Later, after seeing more of the pods being sent to neighboring towns, Kauffman and Jack, who have been replaced, tell Bennell the aliens' plan to make like more simple by getting rid of emotions. Becky later falls asleep and is replaced, causing Bennell to take off running to a nearby town to warn them of the impostors. This is when he is arrested and when the first scene takes place.
In this film, the aliens destroy humanity not by killing them or destroying their cities, but by getting rid their emotions. Many other cold war era sci-fi films show the end of the world by communism via nuclear weapons and their subsequent radiation. This film goes a completely different direction by suggesting that communism wins when everyone conforms to that ideology. Although the people involved in making the movie denied any purposeful references to the Soviets or communism, there are clear signs of it, intentional or not. The impostors are implied to be more efficient in whatever it is they do, probably as a result of not being bogged down by silly emotions. They are all the same person inside, but different outside, which is how they fool Bennell and co. Because of the unique threat posed in this movie, it holds up better today because there isn't as much competition. There are so many nuclear apocalypse movies, games, tv shows and such, but not a lot about the world ending by this kind of internal threat.