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, Isabella. But when she suddenly disappears his desperate search for her takes him to a netherworld of slum-dwellers, pickpockets and scavengers of subterranean London.