The Vicar - Dr Charles Primrose - lives an idyllic life in a country parish with his wife Deborah, son George, daughters Olivia and Sophia, and three other children. He is wealthy due to investing an inheritance he received from a deceased relative, and he donates the ÂŁ34 that his job pays annually to local orphans and war veterans. On the evening of George's wedding to wealthy Arabella Wilmot, the Vicar loses all his money through the bankruptcy of his merchant investor who has left town abruptly.
The wedding is called off by Arabella's father, who is known for his prudence with money. George, who was educated at Oxford and is old enough to be considered an adult, is sent away to town. The rest of the family move to a new and more humble parish on the land of Squire Thornhill, who is known to be a womanizer. On the way, they hear about the dubious reputation of their new landlord. Also, references are made to the squire's uncle Sir William Thornhill, who is known throughout the country for his worthiness and generosity.
A poor and eccentric friend, Mr. Burchell, whom they meet at an inn, rescues Sophia from drowning. She is instantly attracted to him, but her ambitious mother does not encourage her feelings.
Then follows a period of happy family life, interrupted only by regular visits of the dashing Squire Thornhill and Mr. Burchell. Olivia is captivated by Thornhill's hollow charm, but he also encourages the social ambitions of Mrs. Primrose and her daughters to a ludicrous degree.
Finally, Olivia is reported to have fled. First Burchell is suspected, but after a long pursuit Dr. Primrose finds his daughter, who was in reality deceived by Squire Thornhill. He planned to marry her in a mock ceremony and leave her then shortly after, as he had done with several women before.
When Olivia and her father return home, they find their house in flames. Although the family has lost almost all their belongings, the evil Squire Thornhill insists on the payment of the rent. As the vicar cannot pay, he is brought to prison.
Afterwards is a chain of dreadful occurrences. The vicar's daughter, Olivia, is reported dead, Sophia is abducted, and George too is sent to prison in chains and covered with blood, as he had challenged Thornhill to a duel when he had heard about his wickedness.
But then Mr. Burchell arrives and solves all problems. He rescues Sophia, Olivia is not dead, and it emerges that Mr. Burchell is in reality the worthy Sir William Thornhill, who travels through the country in disguise. In the end, there is a double wedding: George marries Arabella, as he originally intended, and Sir William Thornhill marries Sophia. Squire Thornhill's servant turns out to have tricked him, and thus the sham marriage of the Squire and Olivia is real. Finally, even the wealth of the vicar is restored, as the bankrupt merchant is reported to be found.

Question: Why did George challenge Thornhill to a duel?
Answer: He found out that Thornhill was evil.
Alternative answer: He'd heard about all the evil Thornhill had inflicted on his family.