Christine and Jacques Coustard are blown up and killed in their Paris home before they can testify against gangster George La Force and La Force makes sure that he is acquitted at his trial by having the judge's family held hostage. The Saint arrives to avenge his friends the Coustards,posing as a jewel thief named Lamont,who,in order to reel in La Force,interests in him in the Blue Dulac,a priceless Russian necklace that the Saint intends to steal and pin the blame on La Force.