Young poetess Rose Elliot buys a book from a local antique dealer, a diary in Latin of an architect, E. Varelli. She learns of the Three Mothers, and believes her apartment building is one of their houses. She pleads her brother Mark, who is studying musicology in Rome, to come, because she is afraid. Mark's friend Sara reads her letter, which he left behind in class, and discovers the school is run by the Mater Lacrimarum, and is killed for this knowledge. The house of Mater Suspiriorum has already been destroyed, and by the time Mark arrives in New York City, he is investigating his sister's murder.

A deranged killer lives in the basement of an old mansion and pops out occasionally to commit grisly murders that include be-headings, ripped throats, and stabbings with a fireplace poker. The killer needs fresh body parts to rejuvenate his cells. He also has maggots for blood.

With Argento's trademark visual style, linked with one of his more coherent plots, Tenebrae follows a writer who arrives to Rome only to find somebody is using his novels as the inspiration (and, occasionally, the means) of committing murder. As the death toll mounts the police are ever baffled, and the writer becomes more closely linked to the case than is comfortable.