Just to jog your memory (mine to) I found this at Frank's Reel Movie Reviews:
The movie begins with Ronald Wilby (Scott Jacoby) opening his birthday presents. But there is no one attending his party - no one except his feeble mother (Kim Hunter) clad in a June Cleaver dress and white pearls. We learn that Ronald doesn't have any friends and we see that his mother takes a very active, but smothering, role in his teen-aged life. She has been divorced for many years from her husband with whom she struck a divorce deal stating that she will not seek alimony and child support if the father promises to stay out of their lives. Very much like in Sybil, and Carrie, Ronald's mother takes on the all-encompassing role of mother, father, friend, peer and Ronald's sole contact to the outside world. She compensates for her own social inadequacies by inflicting ridicule and maternal dominance upon him - then repairs the damage with hugs, praise and coddling.
We feel Ronald's humiliation as he awkwardly asks a girl for a date. She not only rejects him but joins her friends in taunts and ridicule. In his rage, he shoves one of the girls, who subsequently hits her head on a rock and dies. In a fit of panic he buries her body and returns home.
He and his mother wall off a spare bathroom under the stairs and decide that Ronald will live in the room until the police investigation cools down. They install a trap door under the pantry for access and concoct a series of knocks as a crude means of communication. "Two knocks for danger, four for safety". But their plans go horribly awry after Ronald's mother dies in an emergency surgery and the house is sold to an unsuspecting family. Now the fun and games begin.
Ronald retreats into a psychological fantasy world and begins to wreak havoc on the new occupants of the house, drilling holes through the plaster board walls to keep an eye on the young daughters. The concept of a stranger "living in the woodwork" is extremely creepy and we realize that it is not really that far-fetched. I found myself sympathizing with Ronald, perhaps because we now understand how critical a child's early parental tutelage is to forming what he will become later in life. We all have experienced some degree of humiliation in our lives, making it quite easy to find an emotional connection with Ronald.
The acting is mostly B-movie quality, although Kim Hunter stands out in her disturbing portrayal of Ronald's doting mother. We are also treated to the performance of a very young and much slimmer Dabney Coleman as the father of the house's new inhabitants. It's hard to ignore the high "kitsch" factor of the production with its cheap sets and poor lighting, but the movie's premise alone is the star of the show and is guaranteed to keep you awake for a few nights. There are a few unexpected twists and Ronald's fantasy world is a nice subplot that winds its way throughout the movie. Overall a great horror movie that will definitely scar your psyche.
Oh, that movie must be remade!