A shadowy pursuer seems to be tracking Martha Sayers Terry Lumley, as she drives to her sister’s lakeside house in Los Angeles. On the same day, the police and Martha’s sister Elizabeth Pamela Franklin find Martha hanged in the living room. The authorities label it an unmotivated suicide, but Elizabeth refuses to accept that verdict and begins a careful, relentless inquiry.
Seeking answers, Elizabeth travels under an assumed name to the exclusive Salem Academy for Women, where she hopes to uncover the truth behind her sister’s death. She is welcomed by new classmates Roberta Lockhart Kate Jackson, Debbie Jones Jamie Smith-Jackson, and Jody Keller Cheryl Ladd, though the atmosphere is oddly tense and protective of secrets. Elizabeth’s investigation is initially met with warnings from the feared headmistress Mrs. Jessica Williams Jo Van Fleet and guarded silence from the faculty.
Strange occurrences begin to mount. Debbie has an outburst in class, adding to the growing sense that something is very wrong at the academy. When Lucy Dembrow Gwynne Gilford commits suicide, Elizabeth redoubles her efforts. Her attention fixates on a painting of Martha set in a dungeon-like room; Debbie, who painted it, insists she invented the background, yet Elizabeth discovers an identical setting in a hidden cellar on campus. A tense encounter with a man wielding a straight razor shakes Elizabeth, and she hastily withdraws.
The following night, Elizabeth returns with Roberta, who grows increasingly suspicious herself. They uncover a concealed chamber in which Debbie’s body is found, confirming their fears that murder and manipulation are entangled with the school’s doors. Elizabeth suspects Professor Delacroix Lloyd Bochner because of his peculiar, mildly sadistic demeanor. She storms into his office and finds him in a deranged state, gun in hand; he fears something supernatural stalking him, and he jumps through a window, only to be dragged into a swamp where he is beaten by several students, including Jody.
At the center of it all stands Dr. Joseph Clampett Roy Thinnes, who reveals to Williams that he is responsible for the murders. He orders the headmistress to evacuate the school, but Elizabeth and Roberta remain unaware of the plan. They later encounter the murdered Delacroix’s body, a brutal reminder of the danger surrounding them, and Williams appears to have slipped into insanity.
With no help from the authorities, Elizabeth and Roberta descend into the cellar in search of a weapon. Instead, they walk into Clampett’s shocking scene: a Satanic cult led by him, with Jody among his fervent followers, and Roberta revealed as his loyal servant. Elizabeth manages a perilous escape by setting the academy on fire and fleeing with Williams in tow, while the other students stay behind as the building fills with flames. Clampett miraculously survives the blaze and reappears outside, only to vanish into the night, leaving behind a single patch of charred grass where he once stood.
