A broad picture of Sir Walter Scott's The Bride of Lammermoor might start with the comment that this story is perhaps Scott's answer to what would have happened to Juliet if she had been forced to marry Paris. It is a similar story to Shakespeare's Romeo and Juliet in that there are warring families and youths who accidentally meet and fall in love. One of the differences is that the young lady's father encourages the match, while the other is that fate conspires to force the marriage of the young lady to a man of her mother's choosing. The third difference is that while Juliet loses her life in the end, Scott's heroine loses her sanity in the end.
Edgar, Master of Ravenswood, is embroiled in an inherited hatred of Sir William Ashton. The Glorious Revolution was the cause of the Ravenswoods losing their land and of Sir William Ashton gaining those lands. Hence the enmity. Edgar saves the life of a young lady only to find out that he has quite by accident saved Lucy Ashton, Sir William's daughter. Never mind, they fall in love anyway, family feud notwithstanding (like Romeo and Juliet).
A change in political powers makes Ashton choose to reconcile with Edgar and encourage the love between Edgar and Lucy. But Lucy's mother has other ideas and forces Lucy's marriage to another, to Bucklaw. Later that night, after Edgar crashes the ceremony, too late to stop it, and challenges Lucy's husband and brother to a duel, Lucy tries to murder her husband and is driven mad with by the tragic consequences of deprived love.