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The Oldest Law was adapted from The Price She Did Not Pay, a play by Romena Rue. Fresh from the mountains, Jennie Cox (June Elvidge) tries to land a job in the big city, but within a few weeks she is down to her last three bucks. Jennie is rescued by society man Billy West (John Bowers), whose wife Cora (Eloise West) has just filed for divorce. Promising to adhere to a strict "hands off" policy, West hires Jennie. The whole set-up looks suspicious at first, but soon Cora is accepted within West's circle of friends. Out of devotion to her employer, Cora offers to become the mistress of a gambler to whom West is heavily in debt. Fortunately, Cora is saved from disgrace by, of all people, West's ex-wife Cora, who squares her former husband's debts and graciously gives her blessing to West's inevitable marriage to our heroine.