The episode begins and Sherlock is shown having a good time with two women who are scantily dressed. Next they are shown handcuffing him to a chair. Sherlock plays along as he assumes it’s a part of the fun. But, turns out it’s not, as the moment he is handcuffed they begin robbing him. Just then, cops arrive and arrest the two women, thus reveling that it was they who were set up. The next day he practices whacking a dummy with a "single stick". Watson in the meantime mentions that she has to go, check on the person she has sublet her apartment to.
Later, she talks to her landlord about an issue with the radiator in the apartment and he in turn mentions there was a porn film that was shot in the apartment. He as a result wants to evict her. Watson is visibly shocked at the suggestion and confronts the man she sublet her apartment to. The man admits to her that he was forced to make a porn film in the apartment as he needed the money to pay the rent. He reveals that his career in documentary filmmaking isn’t earning him any money. A patient on the other hand is seen arriving at a hospital. He is apparently there to donate one of kidneys to his sister. Also, he is in cuffs which are removed by the cops before he is taken in for surgery. The moment his cuffs are removed he makes his intentions clear by grabbing a scalpel and attacking the hospital staff. While at it he also kills the guards who had brought him there.
He then makes a makes a run for it. It is then revealed that the man is serial killer. Watson and Sherlock arrive at the scene and check out the carnage the killer has left in his path. Also, he has written the words “Shedir” in blood on the OR door. Sherlock figures that it's a name for the star Cassiopeia. Ennis the serial killer was put away the first time round thanks to a profiler named Katherine Drummond, who is brought in on the case once again. Katherine had written a book about Ennis and was sued for it, as in the book she had suggested that Ennis was sexually abused by his parents as a child. Later, the case against her was dropped. Sherlock it turns out has worked with her earlier on the other case of another serial killer, but apparently doesn’t have very high regards for her.
Katherine arrives and Sherlock doesn’t seem too happy about it. Ennis’s sister in the meantime is feeling guilty about the fact that it because of her Ennis has managed to escape from the hands of the law. She reveals that Ennis and she weren’t in touch and also mentions how glad she is that Katherine is a part of the case, as she was the one who helped nab Ennis the first time round. Later, Sherlock checks out the model of Cassieopeia and concludes the killer has chosen his victims base on a pattern in it. Sherlock also admits to Watson that he had slept with Katherine the last time they had worked together, which in other words reveals as to why the two have an issue with each other. Ennis in the meantime goes on a killing spree in a convenience store. But, he spares the life one woman, after asking her to take his picture, while he is holding up a newspaper with his face in the front page.
Sherlock on finding out about what Ennis did at the store is baffled as to the reasons behind Ennis’s actions. Watson in the meantime figures out that Sherlock has a problem with Katherine, because she profiled him. Basically, although Katherine did not mention his name, she referred to him in a psychiatric journal as “the deductionist”. Sherlock felt betrayed by her actions. In addition, she also mentioned in her piece his struggles with addiction and how that would result in a bad end for him. Gregson in the meantime is frustrated about the fact that Katherine isn’t proving to be too much of a help as a profiler. Ennis calls up at the station and has a chat with Gregson. During the conversation he brings up Katherine’s mentions, and its apparent from his words how much he hates her.
Sherlock on his part concludes that Ennis extreme dislike for Katherine is not only because she helped nab him, but also she figured out what he was all about. Ennis is impressed that Sherlock has figured this out. Ennis reveals he too is aware that Sherlock is “the dedcutionist” who has been mentioned in the journal. Ennis knows this because he has read everything that Katherine has written till date. He is hell bent on teaching Katherine a lesson, as it’s because of her that his family is ruined. He also mentions that his father was a good man and didn’t sexually abuse him. His father killed himself and his mother died shortly thereafter thanks to what Katherine had written, Ennis further reveals. Ennis makes it clear to Gregson that he would stop the carnage and return wants simply wants Katherine.
Sherlock on his part thinks that Ennis’s demand really isn’t too unreasonable. Later, Sherlock confronts Katherine about what Ennis has revealed and expresses his doubts about whether Katherine simply made up things about Ennis. He later discusses this issue with Watson, who mentions that Katherine’s profile of him didn’t all come true and hence it’s possible that Katherine might have made a mistake even with Ennis. Watson sees that Sherlock is watching the porn film that was made in her apartment, but it’s not for titillation. Turns out, he is intrigued by the continuity errors in it. Later, Sherlock and Watson respond to a tip about Ennis having broken into his sister’s house. At her house, the duo discovers that it’s filled with food that no person with kidney problems would ever have. They conclude that Ennis’s sister is in cahoots with Ennis.
Basically, she was as enraged as Ennis that their parents died thanks to what Katherine had written and as a result, just like Ennis, wanted revenge. She therefore agreed to consume poison on his suggestion, so she could be taken for a kidney transplant, thus allowing him to escape, on the pretense of wanting to donate her, his kidney. Katherine’s guilt in the meantime gets the better of her and she pays Ennis’s sister a visit, in order to apologize to her for the allegations she had made in her book. His sister isn’t in a forgiving mood and as a result attacks Katherine with scissors. In the end Katherine survives the attack and Ennis’s sister is placed under arrest. Later, Ennis gives Gregson a call once again. Sherlock in the meantime using his guile manages to trace the source from where the call is coming, despite Ennis doing his best to camouflage it.
He arrives at the location where Ennis is. Sherlock then manages to outsmart Ennis in a battle of wits. The cops later arrive and place Ennis under arrest. Next, it shown that Sherlock has figured out from the continuity issues in the porn film made in Watson’s apartment that, her landlord was there when the film was being shot. In other words he knew about the film being made in her apartment and allowed it. But, Watson shall still be moving out and the landlord shall be paying to put all her stuff in storage, until she finds a new place. Also, he shall buy her a brand new couch, considering the deeds that were done on the old one. The episode ends.