In the early nineteenth century, Irish lass Mary Yellan, upon the death of her mother who was her last surviving relative in Ireland, travels to the Cornish coast to live with her maternal Aunt Patience and Uncle Joss Merlyn at Jamaica Inn, where they are the proprietors. Mary has not seen Aunt Patience since she left Ireland when Mary was a child, and brutish Uncle Joss who she has never met. Despite Mary having written about her imminent arrival, they are surprised when she does show up having never received the letter. Mary's plan to stay with them throws a slight wrench into Joss' activities of which Patience, in her love for him, is aware but in which she is not involved: he and a bunch of other men are involved in causing shipwrecks by hiding the beacon light during stormy weather, they not only pilfering the ships of their valuable cargo but murdering all the surviving shipmates so that they cannot report what happened. While the men see Joss as their ringleader, there is another man who is the secret mastermind pulling the strings behind Joss. In secretly witnessing the men accusing one of their own, relatively new to the gang Jem Trehearne, of holding back some of the stolen goods for himself, they threatening to murder him, Mary begins to learn that Uncle Joss is involved in something criminal, she believing purely smuggling not aware of the murder. Mary believes that Patience and Joss' neighbor, Justice of the Peace Sir Humphrey Pengallan, who helped her in being stranded when she first arrived, can protect her against whatever Joss and the other men at Jamaica Inn are doing. However, Sir Humphrey and Jem are not who or what they appear on the surface, Mary getting involved with them which may place her life at greater risk.