London of the late 19th century is a haven for political exiles of all sorts - refugees, partisans, anarchists. Verloc has made his living spying for the Russian goverment, an agent provacateur of sorts, while simultaneously providing information to the London police, specifically Chief Inspector Heat. When the new Russian ambassador demands he prove his worth or lose his salary, Verloc sets off a tragic chain of events that involves his pretty young wife Winnie, her retarded brother Stevie, and a figure called the Professor, whose fascination with explosives and destruction makes him the person to call on when Verloc needs a bomb.
