It's telling that a show about a 47-year-old American pope with a fluctuating accent and troubling proclivity for Cherry Coke Zero was only the second weirdest episode to air on TV on Jan. 15.

The Season 4 finale of BBC's Sherlockopened with an abandoned plane, the ghost of a child and clowns in an old Victorian mansion. The latter, at least, turns out to be a melodramatic staging, but by the end of "The Final Problem" you long for the simplicity of a false hallucination.

Episode 2 ended with the reveal that Sherlock and Mycroft have a sister, Eurus (Sian Brooke), who Sherlock barely remembers to the point that Mycroft's manipulation has all but eliminated the memories. Eurus was a genius "beyond Newton," (possessed of more than just "the deduction thing," as John eloquently describes it). She "doesn't just talk to people, she reprograms them." It sounds dubious -- because it is.

After childhood actions worthy of Tom Marvolo Riddle, Eurus is sent to Sherrinford -- a fortress at sea for people so unpredictable they can't be near society. Mycroft maintains that it's not an institution or asylum, and uses the word "incarceration" when they break in later.

What the episode struggles to deal with -- mostly by not dealing with it at all -- is that Eurus's existence and role in Sherlock's life fundamentally changes who the character is. He -- and Mycroft, who just happens to remember -- is living with and quelling monumental childhood trauma.

And of course, "The Final Problem" marks the triumphant return of Andrew Scott as Moriarty (obnoxiously triumphant in an introductory helicopter landing set to Queen's "I Want To Break Free" which only Scott could get away with).

Yes, because Eurus's pivotal Christmas present five years ago was not her gorgeous Stradivarius violin, but an unsupervised meeting with James Moriarty during which they plotted the final problem for John Watson and the Holmes brothers. It should be a formidable alliance, but Eurus just feels like a repackaged Moriarty, an attempt to extend the show's best villain to date with more of the same. It's hard to bring something new to a table already sagging beneath its own weight; she's a charismatic anarchist who treats people like playthings, and both the audience and Sherlock have dealt with this before.

The titular problem itself is barbarically convoluted; It tangles together pieces of the past -- the Holmes' siblings childhood, Eurus and Moriarty, an old murder case -- with current events at Sherrinford and the little girl trapped in an unmanned airplane, sipping her free juice box like it isn't probably the last thing she'll ever drink. Freeman described it as "the most traumatic" episode the show has ever done, yet at the conclusion of the 90-minute chapter it's all glossed over; The Baker Street Boys bear no apparent mark of the innocent lives lost in Eurus's game and the psychological toll of how many times they could have killed or been killed.

The most gripping of the escalating obstacles in the episode is Sherlock having to admit he loves Molly, something she deserved to hear ages ago even if it would have been the worst detriment to her emotional growth. It's painful to witness, worsened by the devastating conclusion that Eurus is just making a point about emotional context (right out of the Moriarty Season 1 playbook, using John as bait to compromise his rival's mental faculties).

At many points, "The Final Problem" is a glorified Sawmovie, the torture porn supplanted by polished accents and the BBC's prestige. There are too many moments evocative of other stories, which has never been a problem for the show (is Sherrinford Azkaban? Does Mycroft live in the manor from Black Mirror's "Play/Test"? How would the Joker have streamlined this scheme?). It's saturated with twists, which doesn't lessen the efficacy of the Redbeard reveal but also doesn't stop it from making you physically ill. The identity of the girl in the plane is exhausting by the time it comes around -- it's going to take some processing from both sides of the screen.

Garbled though they are, the episode has its moments: Sherlock doing classic deductions over the coffin, the brief scene between Mycroft and Moriarty, every second of Mrs. Hudson, the music swelling when Sherlock has to choose between killing his brother and best friend, and Sherlock getting Lestrade's name right for the first time.

Mary Watson's addendum to the finale feels like a eulogy for the departure we suspect but can't confirm -- the end of Sherlock. Even as it leaps through stylization and ostentatious writing, the show's glowing core has always been the two men who anchor it, and whose talent and demand ate away at the show's schedule. Season 5 may never happen -- but there's still a chance it's not the end.

That's a problem we can live with.