Michael Caine stars as an espionage agent whose young son is kidnapped. Complicating matters is the fact that the kidnappers (John Vernon and Delphine Seyrig) are Caine's own colleagues. They want to secure Caine's aid in rounding up a diamond smuggling ring, and they don't care who they have to hurt to do so.
He agrees to go along, all the while searching for his missing son. Janet Suzman co-stars as Caine's estranged wife, who is compelled to join him in his search. Helmed by veteran filmmaker Don Siegel, The Black Windmill is based on Seven Days to a Killing, a novel by Clive Egleton.