British agent Major John Tarrant's son is kidnapped by an international criminal organization demanding diamonds as ransom. Tasked with locating the perpetrators, Tarrant discovers that the ransom demands are intricately tied to an operation involving his own agency. Facing suspicion from both his superiors and the criminals, Tarrant finds himself isolated and determined to rescue his son. His quest takes him across Europe, confronting a web of espionage, betrayal, and constantly shifting alliances. His only clue is the name "Black Windmill," a crucial element in the mystery he must unravel to bring his son home.

As Tarrant delves deeper, he uncovers a conspiracy that reaches high into the corridors of power. The dual pressures of time and mistrust lead him to question not only his colleagues but the very principles he once believed in. With every step closer to his son's captors, the stakes grow higher. Seemingly insurmountable obstacles force him into a dangerous cat-and-mouse game, where every move is a matter of life or death. Unraveling the connections between the ransom, his agency, and the enigmatic Black Windmill, Tarrant must navigate treacherous waters to save his son and uncover the truth behind a plot much larger than he initially imagined.