Cunningham is a troubled young man who is taken in by the Marquis of Montrose, in hopes that the climate of Scotland "will cool the fever in his blood." Cunningham is trouble from the very beginning, immediately seducing the young housemaid, Betty, working in the Marquis' home, and spending the Marquis' money without care or reason.
One man who plans on stretching every last guinea is Robert Roy MacGregor a.k.a. Rob Roy, a local Scot trying to build up his lands and to make a home for his people. He contracts with the Marquis for a loan of 1000 guineas, but when MacGregor's man disappears with the money, treachery is the first thing on his mind. It is, of course, Cunningham and the Marquis' man Killearn who have conspired to steal the money, and Cunningham does more than that: he kills one of MacGregor's dearest friends and dumps the body in the nearby loch.
Besides being a murderer, Cunningham is a violator of women, usually hiding behind his station to persuade or force the opposite sex to do his will. In the case of Betty, she falls in love with the young man, and he cares nothing for her. Even after she reveals she is pregnant by him, her dear "Archie" tells her with a smile: "Love is a dunghill, and I am but a cock who climbs atop it to crow." When the legendary Rob Roy is declared an outlaw by the Marquis, Cunningham greedily takes on the job to bring him in. He heads straight for MacGregor's home where his wife Mary remains behind. Cunningham orders the family's cattle slaughtered, their crops destroyed, and their house burned down. In the middle of it all, he rapes Mary while Killearn watches. He exits with a grin and makes sure Mary knows his name so the great Rob Roy can come for his revenge.
Cunningham eventually does capture MacGregor; he ties him up and beats him savagely, completing MacGregor's humiliation by dragging him behind a galloping horse for Cunningham's amusement. Rob Roy escapes only to learn the terrible truth about what Cunningham did to his wife, and he arranges a duel with the villain through the help of a local nobleman who stands against the Marquis. Cunningham battles to certain death, MacGregor, a brawler with the sword, is continually nicked and stabbed by the swifter Cunningham until he can take no more. As Cunningham holds his sword at MacGregor's neck and waits for approval to finish him off, MacGregor grabs the end of the blade before slicing Cunningham deeply across his torso, nearly cleaving him in two, and avenging Mary in the process.