Two strange sisters live in a crumbling mansion, where they keep a pet ape, which belonged to their late father, locked in a cage. While one of the sisters seems to be keep her head on straight, as it were, the other appears to be sinking further and further into barbarism and insanity.

After their anthropologist father dies, leaving them alone in Africa, Ellen has the responsibility of taking care of her mentally ill sister Cissy. Cissy amuses herself by drawing pictures of the primates she keeps in cages and then killing them. Ellen is an astronomer, and longing for some normal companionship, she begins a romance with her co-worker David. Soon after this, Ellen leaves on a business trip, leaving a family friend to look in on Cissy. But when David, who is curious about Ellen's home life, comes to see Cissy she becomes unhinged, resulting in tragic consequences.