Wow. As far-fetched as fan theories about this season of Sherlock were, I don't think anyone foresaw anything close to this demented carnival of a finale. Not only did the plot surprise at every turn; even the very style of the show shocked and upended expectations. In what may be the final episode of the show, everything coalesces into an astoundingly extreme parable of the dangers of Sherlock's purported strength: cold-blooded reason. If unleavened by any sort of emotion or love, it can cause a horrible and terrifying absence of basic humanity in a person.
The episode starts off in a horrible and terrifying manner--cinematically, that is, not morally. After a quick set-up (all the passengers on a plane are sleeping or dead while a lone girl is left to survive, her only lifeline a cell phone with Moriarty's voice on the line), Mycroft is plunked into a horror movie. Haunted young girls, creepy sing-song voices, bleeding portraits, and a grotesque clown all stalk him through a Gothic mansion. Is this his mind palace? What's going on?
It's Sherlock of course, stoking Mycroft's fear of the newly revealed Holmes sister, Eurus, in order to scare him into admitting that Eurus exists. In short order, almost all of the questions brought up by last week's episode are answered. Eurus shot John with a tranquilizer, not a live bullet, at his therapy session. The lost Holmes sibling has been held for years by Mycroft at Sherrinford, a maximum-security prison that Mycroft memorably describes as the "map reference for hell."
Eurus is even more brilliant than the other Holmes siblings, an "era-defining genius." But she seems to lack basic human features. She doesn't understand pain. She cuts herself to see how her muscles work. She traumatizes her brother Sherlock by making his beloved dog Redbeard disappear and giving Sherlock a song to find Redbeard that he never solves. She burns the ancestral home down, and that's when she is sent to Sherrinford. Oh, and Sherlock doesn't remember her at all: Mycroft has blocked Sherlock's memories of her.
We then shift from horror movie to spy thriller. (This is not the familiar mystery Sherlock). A drone with a grenade alights in Baker Street, singing Eurus's song about Redbeard. The detonation on the grenade is motion-activated. Sherlock, John, and Mycroft all leap from the flat as it explodes--it's hard to tell whether the gaudy fire effects are parody or serious. And we're off to break into an island fortress guarded by machine-gun-armed men in the pouring rain.
Mycroft and John are captured while Sherlock breaks in to see Eurus in her cell. The plan quickly goes awry. Mycroft and John learn the governor of Sherrinford has disobeyed Mycroft and let psychologists evaluate Eurus. But she's so compelling and brilliant that she can "enslave" whomever speaks with her, as with the doctor she "reprogrammed" to kill himself and his family. Or the governor and the entire prison, which means she controls Sherrinford. And that explains why her cell has no walls, letting her ferociously attack Sherlock.
Then Moriarty finally makes his comeback, to the jaunty saunter of Queen's "I Want to Break Free." His helicopter alights at Sherrinford--but this is in the past. He truly is dead. Yet five years ago, he was Mycroft's "Christmas present" to Eurus, who the British government occasionally uses as an asset: she predicted three terrorist attacks from spending an hour on Twitter, but requires "treats" in return. A Stradivarius violin was one, and five minutes of unsupervised conversation with Moriarty was another.
Back in the present, the episode shifts gears again into a Saw-like thriller. Sherlock, John and Mycroft must complete tasks in order to save the girl on the plane that we saw at the beginning of the episode. First up, John or Mycroft must shoot the governor of the prison, or his wife will die. Both refuse, though John comes close when the governor appeals to his own sense of marital duty. The governor shoots himself, and Eurus shoots his wife. She tells Sherlock to take the gun and move onto the next task, and the trio had better not let their moral code get in the way again. In Eurus's hellhouse, morals only make things worse.
Sherlock must now solve a murder case from the scant evidence of the gun used to kill the man and three photographs: which of these three brothers did it? To make matters worse, videos of Moriarty up the anxiety. To really make matters worse, the three brothers are hanging outside the window, tied up and dangling over the ocean. Whoever's the culprit dies. Actually, once Sherlock solves it, Eurus drops the two who didn't do it. Don't like that? She drops the third man too. What's a life to her? Killing people makes her feel nothing.
On to the next room. Deduce the owner of the empty coffin, labelled "I love you." Now save that owner--Molly Hooper--from the explosives rigging her apartment by calling her and getting her to say "I love you" before the clock is up. This whole "complete tasks" thing is some of the most intense television I've seen, and this sequence in particular is devastating. At this point, you're ready to believe the show capable of anything, even killing the sweet-hearted Molly. Sherlock manages to get Molly to say the disarming phrase after an emotional wringer of a call, and we learn that there were no explosives. Eurus is simply turning emotion on her brother, attempting to illustrate the folly of having any feelings at all.
But worse is to come. Now Sherlock must use his final bullet to shoot John or Mycroft, his choice. Pushed to the breaking, he figures out how to cheat the game. He points the gun at himself. Eurus freaks and tranquilizes Sherlock and John.
When they wake up, John is chained in a well with the bones of a human child, and Sherlock is in a room outside his ancestral home. He still has to figure out how to save the girl in the plane, which is slowly losing altitude, and now he must also save John, who will soon be drowned as the well fills up with water. To rescue his best friend, Sherlock has to solve the first case he ever received: what happened to Redbeard? Those bones in the well? They're the skeleton of Sherlock's childhood best friend, who went by Redbeard when the two boys played pirates. Mycroft has dulled the blow of this early trauma by telling Sherlock that Redbeard was a dog. But Eurus actually killed a boy, not a dog, because she wanted to play with them and felt she had no one in the world.
More upendings: the girl on the plane is actually Eurus, trapped in her mind with no one to help her and starved of love. Sherlock finds her in the mansion and proves she is not alone, for her brother is there for her. John is saved. Eurus is sent back to Sherrinford. She and Sherlock duet on the violin. Baker Street is reconstructed. And Mary provides a moral wrapping-up of the entire show in a voice-over from her video to Sherlock. Sherlock and John will always be there for the lost and dispossessed. They are the last hope, even for someone like Eurus who is morally stunted by their disposal of feeling in favor of reason. We finally know for sure that Sherlock is not that emotionally starved, cooly logical person. He's not pure reason. He has sentiment, love, and John.