Although _Shock_ is not a perfect horror movie, it has great moments, and must have inspired other haunted house/ghost stories - not the least of which is _The Amityville Horror_, yes this came out first. I would also believe that _Insidious_, _The Babadook_, and several Japanese horror movies owe some amount of inspiration to this.
The story is very simple: a mother (Dora), son (Marco), and stepfather (Bruno) move back into the house previously occupied by Dora, and her first husband, who died there. There are twists that will come as no surprise to anyone who has ever watched a horror movie.
The focal point and real star of the film is Dora (played by Daria Nicolodi) - who is gorgeous and looks fantastic in her '70s clothes - she is marvelous as a woman slowly coming unhinged as the events intensify.
Her son, Marco, is an annoying little kid. That's it, what else do you want me to say?
Bruno is the typical horror-movie skeptic - someone has to be there to push our heroine or there would be no movie.
The overall look and feel of _Shock_ is what one would expect from an Italian horror movie from the era, meaning it's very drab and autumnal.
Are there parts of the movie that don't make sense? Yeah, but for the most part it's great. Is the music hit or miss? Yeah, but aside from the theme that sounds like an ice cream truck, another that sounds like psychedelic rock, and one that sounds like a camp counselor playing an acoustic by a campfire, it's pretty good.
I'll restate that _Shock_ is not perfect, but this is really a hidden gem. Bava is one of the all-time greats for horror, and I'm surprised this is not mentioned more often when people talk about influential horror movies.