The lingering question for Sherlock fans — every year, it seems — is whether the series will return for another season. Indeed, it's difficult to mitigate when there's several years in between seasons, but if fans just witnessed the final episode of this iteration of the famous detective, the show ended on a positive and fulfilling note.
Episode three of season four, "The Final Problem," pitted Sherlock, John Watson and Mycroft against the Holmes family's long-lost sister, Eurus. Why Sherlock didn't remember her, and where she'd been this whole time, were questions the show needed to address; lest Sherlock divulge into its increasingly farcical logic (see: the entire plot of "The Abominable Bride"). Thankfully, Mycroft provided a somewhat believable answer: Sherlock had forgotten about his sister, the result of the trauma he endured as a child from her.
We get it in bits and pieces at first, and yes, she sounds awful. She drowned the family dog, Redbeard, who Sherlock was attached to, and subsequently set their house on fire. It was then that Mycroft faked her death for the sake of his parents and covertly sent Eurus to a very isolated facility called Sherrinford. It's essentially Alcatraz on steroids: an isolated supermax prison in the middle of the ocean where the worst people imaginable are kept. That's where Mycroft assumed she was the whole time, though that was obviously called into question earlier in the season when we learned that she had posed as several different people, including John's therapist.
A good chunk of "The Final Problem" takes place at Sherrinford, where we saw not just the best parts of the episode, but also of the entire season.
Mycroft explains that Eurus' extreme isolation doesn't — or at least, shouldn't — include conversations with people because she can "reprogram" them to become her mindless followers. Her capacity for persuasion included convincing a doctor to kill his family, before killing himself. But the facility's supervisor still allowed people to speak with Eurus — including himself. He and the facility become compromised; a chilling example of inmates running the asylum, with Eurus at Sherrinford's helm. This is confirmed with a fun cinematic trick: Eurus, seemingly behind a wall of glass, stretches her hand to meet Sherlock. It was an illusion (and a really clever one), and revealed that she had the upper-hand, while the guards apprehend Mycroft and John.
This scheme ostensibly started when Mycroft gave his sister a "Christmas gift" several years ago: Five minutes of unsupervised conversation with Jim Moriarty, who, while not back from the grave, was a looming, ethereal presence in "The Final Problem." He and Eurus are formidable adversaries on their own, but with five minutes alone, they set up a world of problems for Sherlock from within the walls of Sherrinford.
What followed was an enticing, Saw-like set of puzzles for Sherlock, John and Mycroft to solve. The biggest one featured a little girl in a plane — the only person on the flight awake for some unknown reason — with Sherlock given small windows to chat with her and help her somehow land the plane. All the while, the trio had to solve in-room puzzles from Eurus that led to an ever-increasing body count. In the most gripping sequence, Sherlock has three minutes to chat with Molly and convince her to say "I love you" over the phone, otherwise her apartment would detonate with explosives. Naturally, it came down to mere seconds.
The puzzles continue until Eurus abruptly ends the game — tranquilizing the trio — after Sherlock nearly shoots himself in the head, instead of playing by her rules and choosing to kill either John or Mycroft. It leads to Sherlock finding his way back to the burnt down family home, where Sherlock learns the truth about Redbeard, and the girl on the plane. Redbeard wasn't a dog but a person: Sherlock's childhood best friend, who you might have guessed, enjoyed dressing up as the pirate Redbeard. Eurus left him to drown in a well, which is unequivocally messed up.
The girl on a plane? A metaphor for Eurus' intellect being on a higher plane than everyone else's, thus feeling extremely isolated (OK, that was a bit of a stretch, Sherlock). Unfortunately, this was where "The Final Problem" falls into a brief rut, with the girl on the plane reveal and Eurus' subsequent surrender, which amounts to Sherlock acknowledging that she deserved to have a friend and more attention growing up. Suddenly, she's willing to be apprehended by the police and sent back to a now-secure Sherrinford.
The sentiment from Sherlock is nice, though ultimately implausible considering how quickly he was willing to forgive his sister for — we remind you — drowning his best friend in a well, plus almost getting Molly killed (while killing other characters in the process!). That she's returned to Sherrinford to live out her days satisfied with regular visits from her brother, amounts the entire plot of "The Final Problem" to a bit of inconsequential hogwash.
Any issues with the culmination of the Eurus plot are assuaged by a final, hopeful message from Mary, which, between her and Moriarty, means a lot of characters on Sherlock enjoy recording messages before their deaths. Mary lovingly sums up the reasons why John and Sherlock enjoy solving crimes — for a junkie like Sherlock, the thrill of crime-solving is a natural high, while John still has a war mentality from his service in Afghanistan. "Who you really are, it doesn't matter," she says. "It's all about the legend."
"The Final Problem" ends with a montage of Sherlock and John solving crimes, before a freeze-frame of the duo running out of a building precedes the end credits. It has all the makings of not just a series, but season finale for Sherlock, which would appear to have run its natural course as a TV drama. The increased stardom of its leads, Benedict Cumberbatch and Martin Freeman, would indicate they'd each be hard-pressed to find time in their schedules for another go-around, and from the perspective of the show, it's hard to top what they've already accomplished. No new villains could follow Moriarty and Eurus in scope.
Sherlock diehards will certainly clamor for more, but anything beyond season four would feel like unnecessary fan service. That's not this Sherlock's style — no matter how much Sherlock-John fanfiction is created in their honor.