The story of Là-Bas concerns the novelist Durtal who is disgusted by the emptiness and vulgarity of the modern world. He attempts to escape it by studying medieval topics and plans to write a biography about the 15th century murderer and devil-worshipper Gilles de Rais. Through his contacts in Paris with Dr. Johannes, (modelled after Joseph-Antoine Boullan, a French Roman Catholic priest accused of Satanism), Durtal discovers that Satanism is not a thing of the past, but alive and kicking in turn of the century France. He embarks on an investigation of the occult underworld with the help of his lover Madame Chantelouve. The novel climaxes with a description of a black mass.