A man starting a new life in a new town wakes up at the scene of an accident. His accident. Someone had dragged him out of his car and took off in a hurry just as he was waking up. Or rather, something did. He couldn't make it out properly but he knew its silhouette and its lanky scampering movements could hardly be described as human.
Once the local authorities see him safely to his new house, he tries to officially get his new life off to a good start but is plagued by paranoia leftover from his old life. Afraid that his old life found him so much sooner than he had ever expected, he takes measures to make certain of whether or not he was just imagining things. Unfortunately for him however, he wasn't imagining it. Even more unfortunate, it wasn't his old life coming to haunt him but whatever it was that dragged him out of his wreck the night he arrived.
One by one, the people around him soon begin behaving unusually and anytime he tries to learn more or do something about it the weirdness only spreads farther and wider. One day, pretending to be in on their secret, he follows the weirdos to one of their meetings and learns that the town is falling prey to a race of bodysnatchers that are getting ready to do something big in a few weeks time.
He tries to gather together a resistance under their noses passing off his exterminations as accidents but for every step forward he takes he ends up taking two steps back when it's all said and done. When the big day comes, he is left with very few choices and opts out to start a new life somewhere else and forget about the town. While the bodysnatchers make their big push to take over the remaining half of the town out in the open, the man tries to save whoever he can and trap and kill as many of the monsters he can on his way out.
By the end of it all, the bodysnatchers have won and have the man and his group cornered. In a last ditch effort, the man tries to persuade them into letting them go as they are not all too keen on their threat of suicide. Reluctant to see their bodies go to waste, the bodysnatchers let them leave town in peace and the survivors look for new lives of their own with the man who only just joined their community three months before.
The twist is that the man was already a bodysnatcher but his being in an accident screwed up the consciousness takeover so everyone thought he was actually playing the other side through most of the story.