A long time ago I wrote a review comparing A Scandal in Belgravia. His Last Vow gave me an opportunity to pick back up the old thread. The Charles Augustus Milverton of canon is quite similar to Magnussen despite the changes made.
The CAM of canon is a blackmailer of the worst kind, stealing compromising personal letters and other information. Crooked servants feed him the secrets of their employers. He threatens the elite with ruin unless they hand over significant sums of money. He delights in seeing people suffer social and financial ruin. In some cases he has held onto information for years waiting for the right opportunity to strike.
Lady Eva Blackwell asks Sherlock for his help in dealing with Milverton. She is engaged to the Earl of Dovercourt, but Milverton has in his possession letters to a former romantic interest. The exposure of those letters would break off the engagement. Sherlock proceeds to negotiate with for a quick end to the situation without damage to Lady Blackwell’s reputation. Milverton asks for £7000 or else the letters will be shown to the Earl the day before the scheduled wedding. Sherlock refuses this offer because Lady Blackwell does not have that much cash on hand.
After following Milverton’s movements for a few days, Sherlock forms a plan. He enters Milverton’s house disguised as the housemaid’s fiancée. Sherlock and Watson sneak in and successfully locate his safe with all of the stolen information. In a strange twist of events, Milverton has a guest in another room. The guest turns out to be one of Milverton’s victims She blames him for ruining her life. Before Sherlock could help, the woman shot and killed Milverton. When the cops hear about the murder, Sherlock refuses to get involved because his sympathy is with the victims of Milverton’s foul crimes.
Although many elements remain the same, His Final Vow extended the story in order to conclude the arc of the season. Some changes are also the result of modernization. CAM was mentioned briefly in the previous two episodes. Milverton is now Magnussen, a very powerful newspaper owner and businessman. He is still a blackmailer, but his stakes are even higher. The victims of his schemes are now government officials and even Mary Morstan. Magnussen has dangerous information on her past life as an intelligence agent. Confronting him results in Morstan shooting Sherlock before Magnussen attacked either of them. Sherlock realizes the best way to protect everyone he loves from Magnussen is to kill him.
Modernization is the source of the majority of the changes. Social ruin is not enough considering Mycroft’s proximity to government. Although it would have been satisfying to see Morstan or another victim kill him for revenge, Sherlock’s turn to the dark side is just as dramatic. His sympathy for the victims is enhanced by the change because he vowed to protect John and Mary at any costs. Once Magnussen revealed he wanted Mycroft’s secrets as well, brotherly love factored in as well. Sherlock is a high functioning sociopath but he also has a sense of justice. These changes also compliment this series’ focus on character development over plot. The reveal of her backstory cement Mary Morstan as a complex character. Although her past is murky, Watson’s love and Sherlock’s friendship point her towards a bright future.
I believe reading canon is fundamental to appreciating the many adaptations in media. I hope everyone who reads this review goes back and reads the original story.