If Arthur Conan Doyle were writing it, he might call it “The Adventure of the Changeable Detective.”

“Sherlock” arrived on BBC and PBS in 2010, a fresh and frantic reinvention of Conan Doyle’s Sherlock Holmes tales that focused on the pure joy of intellect. Holmes, played with a waspish glee by Benedict Cumberbatch, delighted in solving puzzles no one else could solve, and we delighted along with him. The bromance between Holmes and John Watson (Martin Freeman) added some emotional texture but didn’t get in the way of the fun.

Then came the show’s third season, in 2014. A little of the air went out of the writing, which was disappointing but not too surprising — Steven Moffat and Mark Gatiss, the show’s creators, had set themselves a very high standard. More ominous, though, was their introduction of a girlfriend and then wife for Watson, Mary (Amanda Abbington), who turned out to have a hidden past as a spy. Her function, it seemed, was to generate situations that would draw out Holmes’s protectiveness on Watson’s behalf — to humanize Holmes. But being inhuman, in a witty and almost balletic way, had always been the best and most interesting thing about him.

Which brings us to Season 4, beginning Sunday in Britain and America (on PBS’s “Masterpiece Mystery!”). It comes after a three-year layoff, except for a Christmas special last year, and begins with an episode called “The Six Thatchers,” based on the Conan Doyle story “The Adventure of the Six Napoleons.” As in the story, someone is seeking out and breaking plaster busts of that titular historical figure.

It’s a transitional, fragmented episode built around narrative red herrings. The mystery of the busts is reduced to a storytelling convenience, and there’s some inconclusive business regarding Holmes’s nemesis, Moriarty (Andrew Scott), who was supposedly killed off last season. And eventually (slight spoiler alert), it becomes another Mary episode, with the tilt toward sentiment and melancholia that implies. (There’s a baby, which allows Mr. Cumberbatch to do some Uncle Sherlock mugging.) Holmes invokes “the game” that’s afoot, but the episode’s real through line is a repeated analogy to the tale of the appointment in Samarra and its theme of the inescapability of death. If there’s a game being played, it’s a grim one.