In the summer of 1849, cholera threatens the city and the people of London. The authorities send millions of gallons of sewage cascading into the Thames - for many Londoners the only source of drinking water.

Joshua Jeavons, a young and idealistic engineer, embarks on an obsessive quest to find the cause of the epidemic. As he labours in a fog of incomprehension, his domestic life is troubled by the baffling coldness of his beautiful bride, Isobella. But when she suddenly disappears, his desperate search for her takes him to a netherworld of slum-dwellers, pickpockets and scavengers of subterranean London.