Lord Glenthorn is a nobleman plagued by an illness, commonly known as ennui. It is an illness of idleness, of disinterest in anything that happens around him. He is in a constant state of boredom and depression. To pass his time he starts to gamble, but this leads to loss of a lot of his fortune. He decides to marry into wealth, and pays barely any attention to his wife.
In his depressed state, Lord Glenthorn decides to find a way to end his life. This is how he comes into connection with his old nurse, Ellinor. He meets her at the gates to his estate. She tells him that she came to see him. At this moment, a terrible accident occurs that leaves him bound to bed, with those around him doubting he will survive. Only Ellinor stays at his side and believes in him. During the time of his recovery, he discovers that his wife will leave him for one of his friends. He tries to persuade her, but they end up divorcing anyway.
Plagued by ennui once again, Glenthorn decides that he needs a change in his life. He remembers Ellinor telling him about his property in Ireland, so he decides to travel to Ireland. The journey is bumpy one, which leaves Lord Glenthorn excited. Upon his arrival to his estate he is met with admiration and praise. He is almost seen as royalty, except for Mr. McLeod, who has a critical view on everything he does. He decides to build a house for Ellinor, and he is generous and soft with people living and working on his estate. Our protagonist makes acquaintances with his neighbors, and falls for the noble Lady Geraldine. But, the love is short-lived as she has eyes for another man.
Lord Glenthorn falls into ennui once again, but it is soon disturbed by riots and break-ins happening close to his property. He takes on a rather neutral approach to this which will result in almost-fatal incident later. After some time, he decides that it might be good to go back to England and bids his servant, Joe, to make preparations for him. Soon, he receives an anonymous letter saying that he shouldn't trust Joe and that he should leave Ireland as soon as possible. He decides to not pay much attention to him, but McLeod tries to advise him otherwise. It is the same night that Ellinor visits him and tells him about the poor rioters' from his estate plans to take him down, Joe is among them.
Glenthorn and McLeod make a plan, and they capture the rioters before they can go through with their plans. All of this excitement makes ennui disappear. The next morning, Ellinor visits again to beg Lord Glenthorn to spare her son, who she believes is among the rioters. He refuses and Ellinor drops a life-changing confession that he is her son and that the actual Lord Glenthorn is Christy. While she was nursing him, she put him in Christy's place to become Lord. Backing this up with evidence, Lord Glenthorn decides to go with what his consciousness demands and confess to Christy about this truth. Christy doesn't want to take his place at first, but after some thought he decides to accept for the sake of his son. Ellinor dies the night before Glenthorn's departure from the estate. He leaves the place an honest, noble and respected gentleman.
Glenthorn, or Christy O'Donoghoe as his birth name is, is getting accustomed to his new life. With a help of his friend, Mr. McLeod, he meets with a respected gentleman called Lord Y, and he encourages him to make a name for himself in the profession of law. He meets Lord Y's family, and falls in love with his young cousin Cecilia. In order to be able to ask for her hand, he is motivated more than ever to succeed in his studies of law. He goes back to London where he discovers that his previous wife was mistreated and died. A few years pass and he returns to Dublin. He makes a name for himself there and is able to make decent money. Encouraged by Lord Y, he proposes to Cecilia, who's been patiently waiting, for him, and she accepts.
O'Donoghoe, formerly Lord Glenthorn, receives a letter from his foster brother Christy, now Lord Glenthorn, who tells him that his son died and Glenthorn castle got burned to the ground. He wants him to take back the fortune and the estate as there is nothing for him there anymore. The book ends with Lord Glenthorn (O'Donoghoe), saying that he is a reformed man free of indolence, and that the castle Glenthorn is being repaired.
