The Grissom Gang (1971)

Barbara Blandish, a young Kansas City heiress, gets kidnapped by some inept local hoodlums for the diamond necklace she is wearing, and then gets kidnapped a second time by the Grisson gang demanding a million dollar ransom. THe Grissons, a family of depraved, ruthless poor white trash, is led by Ma Grissom, a fury whose mustache and house dress clash, whose belief it is that kidnapping plots go awry because the victim is allowed to live. But plans go haywire when her son Slim, a psychopathic, knife-wielder, falls in love with Barbara. Her billionaire father hires Dave Fenner, a cynical private detective, to find her.