The narrator of Tenebrae inhabits a decaying, desolate mansion in the remote and wild countryside with his younger brother and their mad old uncle, driven insane by abuse of opium and alcohol. This nameless narrator is a morbid young man who passes most of his time in a room painted all black, poring over arcane manuscripts dealing with the mysteries of death, while sipping garishly coloured liquors brewed by his uncle or cups of coffee flavoured with arsenic.

When he falls in love with a neighbour, he looks forward to marrying her and trading his life of despondency for one of joy. Perhaps unsurprisingly, though, she finds him rather unpleasant company and instead falls in love with his brother. Driven to murderous jealousy, he resolves upon a brutal crime. But after the consummation of his terrible act, he finds himself haunted by a huge, monstrous spider. Is it a delusion brought on by incipient madness? the reincarnated soul of his murdered victim, returned for vengeance? or does it foretell a fate even more horrifying than can be possibly imagined?