One dark and stormy night, a washed out road strands a group of travelers in a decaying Welsh mansion. Their reluctant hosts, the Femms, are a weird but bizarrely comical family. However, the Femms are terrified of their unseen brother, Saul, who they keep locked in the attic. The storm grows worse and tensions increase throughout the night.
Things come to a head when the lecherous hulking butler (Boris Karloff) attempts to rape the beautiful Margaret (Gloria Stuart). During the struggle, Saul is released. At first he seems a helpless, childlike victim of his family's cruelty but is soon revealed to be a homicidal lunatic fascinated with fire. Saul nearly burns down the house with everyone inside but is killed before he succeeds.
The group leaves the house at sunup.