Not just one, but two!
In 1956, a doctor returns to a small town, where some people have been reporting strangeness, but now everything is fine. Except some people are claiming that other people are not themselves. The doctor finds a friend with a strange growing duplicate, and starts to uncover strangeness. He gets his female friend out, but the pods are being distributed. The doctor has to run away, and is only believed at the end. Perhaps the world will react in time.
In 1978, a health inspector has some friends that randomly encounter strange people saying that people are not themselves. They take time to wander around, not doing much, but taking their damn time to go through the same set of motions that the previous movie did economically, but the big difference this time is that we are in a large city! The inspector destroys a greenhouse of pods, and hides away... and then comes back in a rather stilted fashion, encounters a friend, and .... screech!
The 80 minute movie is good. The 115 minute movie is padded out, just extends the scenes for no reason, just must take longer. And the ending is not a surprise that Donald Sutherland is a pod person at that point, it's plainly obvious, not even trying to be a twist.
At best, you can say the first movie is about small town life, and becoming unsettled as people become alienated. The second movie is about alienation in a big city, where there are lots of people, and connections dirft away. But the first movie has hope that this problem might be corrected, the second movie is just everyone gives into it.
See the first, don't bother with the second.