As Meredith, an academic, stands in a Bloomsbury tobacconist waiting for his two ounces of tobacco, he murmurs a verse of 'London, a Poem' and is astounded when a trap door opens into the London Catacombs, bringing him face to face with the Horton Venus, by Titian. From then on he is trapped in a maze of the illicit art trade, in the company of the redoubtable Jane Halliwell.