At the turn of the 20th century, London has become a haven for political exiles and anarchists. Frequent bomb threats and disturbances interrupt the lives of the city's inhabitants, who live in fear of the terrorists in their midst. One such terrorist is Verloc, a secret agent who is given the mission to strike at the heart of London's pride by blowing up the Greenwich Observatory. But his decision to drag his innocent family into the plot leads to tragic consequences.