The story is, supposedly, based on a "real" event - the author neither denies nor confirms this.
A group of girls, along with 2 chaperons, go on a Valentine's day picnic near a geological formation called Hanging Rock. There is eating, reading of poetry & conversation. A few girls and one chaperon decide to explore the rocky form. Mystery and tragedy ensue, There is mysticism, hints of lesbianism, disappearances and possible criminal activity. It should keep us on the edge of our seats - but it doesn't. The events move like molasses and culminates in an unsatisfying ending.
This is one of the few books which I found less interesting than the movie. It's short but I had a hard time reading it - a bit too melodramatic, characters were irritating and it moves at a snail's pace. It seemed like a short story which was drawn out to be published as a book. My advice? Read the Wikipedia narrative & watch the movie - both are much more interesting.