Daytime TV played this movie sometime 1983-1989. A young preteen boy is the main character (and a girl, his friend of similar age). They individually wander into a unique hot-pink-walled cave tunnel, where they are surgically restrained. A robotic drill implants some kind of object into their neck at nape of head, and renders them part of the group of "others" by some supernatural quality. It makes everyday people part of a larger group of transformed strangers, which may be robot or alien or evil. I didn't understand it too well. But the "group" vs. "Me" dynamic is a theme of te plot, because the boy notices everyone around him with this neck implant and fears them.
I saw this movie on daytime TV in the mid 1980s. The movie is in color. I was a small kid and it was my first "horror/sci fi" exposure so I was so intrigued I can never forget it. I have spent decades thinking about it or trying to find the title.
Also, the entrance to the top of the cave is in plain daylight, it looks like pink bubblegum or fish gills ... I remember this cave's interior lining very distinctly. Thirty something years ago!