As a “high-functioning sociopath,” Sherlock Holmes isn’t a very nice person. Previous seasons of the BBC/WGBH series Sherlock have shown Dr. John Watson to be the central force in the detective’s humanization: a surprising, even risky, development for the character, and one that offered the opportunity for self-awareness, something that Sherlock Holmes — at least the version in the original Sir Arthur Conan Doyle stories — had never bothered with before. This focus on the current iteration of Sherlock as both man and detective has increasingly taken center stage in Sherlock, culminating in the show’s latest season, which recently aired in the United States with three new episodes.

Sherlock’s fourth season had a lot to live up to, but even on its own unconventional terms, it fails to satisfy. (On the terms of a conventional Sherlock Holmes story, it rates even worse.) It’s not that Sherlock suddenly became a terrible show. In fact, much of what made it work in the first place was still intact in its latest season. But the character of Sherlock as seen in Season Four of Sherlock is not really Sherlock. The greatest detective in the history of the western world is no longer doing the thing he was created to do: solve crimes.

Sherlock no longer brings his own cool-headed, insistent moral clarity to the great art and science of crime-solving. Instead, Season Four finds Sherlock parsing family history and getting his head shrunk. Not really — he leaves the therapy sessions to John — but most of the season does seem dedicated to working out his various relationships. This especially applies to the relationships he has with members of his family, which keeps adding members (all of whom have weird names, a joke that Sherlock keeps circling back to in a desperate bid to lighten things up).

For all its psychologizing of its subject, what the final season of Sherlock fails to understand is Sherlock himself.

The season is immediately thrown off-balance by making the character of Mary, John’s wife, central to the first episode’s plot. As a secondary character, she isn’t able to carry the show, especially as she’s not even a detective. Solving a crime is the moral arc at the heart of every Sherlock Holmes story, and when this is not being accomplished by Holmes, viewers are left with a Holmes compromised by other, messier parts of his life.

As the season proceeds, Mary seemingly exits the plot, only to become more present than ever in spirit (if not the flesh). She lingers on in John’s mind, giving him direction and affirming his decisions. She offers a prescience about Sherlock’s every move in a pre-recorded video, which Sherlock uses to consult her even in her absence. The last episode ends with Mary in a voiceover literally having the last word, framing Sherlock Holmes and John Watson, not as world-class crimefighters, but as her “Baker Street Boys.”

The misplaced emphasis on Mary only serves to highlight a more general problem. Over the three episodes, Sherlock becomes a show about relationships. The relationship between husband and wife, parent and child, co-mercenaries, colleagues, two friends, two brothers, two brothers and a sister — all these variations on the theme of “How do I feel about so and so?” become the plot. Solving the crime itself is almost an afterthought.

Unlike the rest of us, Sherlock is unfettered by the ties that bind, a quality that makes him terrifying to normal people but brutally effective when it comes to solving crimes. But instead of watching him soldier on in service of the truth, viewers watch him become increasingly hobbled in each successive episode as he agonizes over the past and his failure to live up to his obligations to the people in his life, from Mary to John to Molly and so on. This turn of events would incite empathy in any other character but it’s bizarre, and worse, unproductive in Sherlock Holmes.

By focusing on Sherlock’s relationships, Sherlock’s writers have necessarily made psychology the focus. This is a burden far too great for psychology to bear. In Sherlock, psychology’s function is to make it possible to understand human motivation; it becomes the master key that unlocks every door. This works, up to a point, but as the scale of the crimes — and criminal pathologies — increase, psychology simply can’t keep up. There isn’t enough science in the world to illuminate what motivates the crimes revealed in the final episode.

In spite of Sherlock’s claim that he is “a brain” and the rest of him “a mere appendix,” the human mind is more than a series of neural pathways just waiting to be understood by science. Psychology, which utilizes science as far as it is able, reaches its outer limits quickly when it attempts to address more unquantifiable aspects of the human experience, such as understanding evil. And Sherlock’s fourth season is about nothing if not evil.

Evil takes the form of Eurus, the sister Sherlock didn’t know he had. Eurus is so terrible that she can’t be imprisoned in a normal facility. Instead, she must be contained, like a comic book supervillain, in a dedicated prison on a rocky island in the frigid vicinity of nowhere, with a bevy of prison guards who must stay three feet away from her glass cage at all times.

The focus of Eurus’s outsized, transcendent evil is her brother Sherlock. He discovers that Eurus was his ultimate nemesis all along, the fountainhead from which all his lesser foes flow. Even Sherlock’s great enemy Moriarty was animated by Eurus; in the end, he was her puppet.

Key to understanding Sherlock’s repression of any memory of Eurus is knowing that she has always been evil, even as a child. Like a malevolent version of the movie Citizen Kane, where discovering the meaning of the word “Rosebud” drives the plot, Eurus directs Sherlock to discover the significance of the word “Redbeard.” Solving this mystery will unlock the shared secrets of their childhood, which is a minefield of psychological misery. Discovering what “Redbeard” means will also save the lives of many people who Eurus will kill if Sherlock can’t solve the puzzle of how things came to be such a mess.

Why is Eurus evil? Viewers don’t know, nor do we care, in the same way we don’t understand how it came about that she is so brilliant. In a show based on bad people who do bad things and good detectives who catch them, that should be enough. But Sherlock takes it upon itself to explain where Eurus’s evil comes from; with psychology determined as the primary engine of human motivation. The story seems to be built on the premise that if Sherlock could find out what happened to Eurus in childhood, he’ll understand why she’s killing people and put a stop to it.

Like his sister, Sherlock is a sociopath, but unlike her, he uses his sociopathic tendencies for good. Sherlock’s detachment provides a moral clarity in distinguishing between right and wrong. Unencumbered by the emotions other people might use to justify a crime, Sherlock can clearly see where criminals go wrong. Conversely, he refuses to use his misanthropic tendencies to commit crimes himself. In Sherlock’s case, detachment is necessary to his work as a detective; without it, his strength is depleted, the equivalent of Samson cutting his hair.

This glaring weakness is revealed in the series’ final episode. As the episode progresses, Sherlock uncharacteristically wades through the morass of emotional entanglements, caught between Eurus and John while desperately trying to discover Eurus’s motivation and frantically searching for her evil’s “off” switch. He must solve the riddles Eurus has placed before him, understanding that he can’t really do so without solving the riddle of Eurus herself. In this effort, Sherlock must be aided by psychology alone, since other, less rational explanations for the origin of her evil are not a consideration. Once, theology or philosophy might have been a help to him, but in Sherlock’s 21st century world, science alone must account for the machinations of both body and soul.

It’s not that psychological damage, such as rejection in childhood (the great secret to Eurus’s crimes), can’t contribute to evil behavior — it’s that it can’t completely account for it. “Who knows what evil lurks in the heart of men?” asks the old pulp series The Shadow. But I know what evil lurks in the heart of men, and so does every other person in the world. Most significantly, God knows: we were born this way.

Psychology may not be able to account for evil, but the doctrine of original sin sure can. Ironically, a foundational understanding of sin, of the inherent imperfectability of man without divine intervention, is necessary to a good detective story, even a rational one. In fact, unaccountable evil is so taken for granted that the best detective stories dispense with the psychological profiling altogether and cut to the chase. Find the clues, solve the crime, bring the criminal to justice, and then retire to the sitting room for brandy and cigars.

For all its psychologizing of its subject, what the final season of Sherlock fails to understand is Sherlock himself. Viewers don’t need to know what makes Sherlock or the criminals tick. We understand that what Sherlock lacks in human warmth, he makes up for in human decency. Sherlock Holmes is a great detective, capable of creating order out of chaos, if only for a moment. But a day will come when justice is finally, ultimately meted out, and humans no longer need a stand-in to track down the perpetrators of evil who are roaming the earth, looking for people to devour.