Like many genre stories, “Sherlock” has inspired reams of “slash fiction” among its viewers, especially its female ones: the term goes back to the homoerotic Kirk/Spock stories of the nineteen-seventies, written pornography in which the shouty captain and the Sherlock-like half-Vulcan went for it. The genre exploded once the Internet came along: you can find slash fic about almost any characters you can imagine, from Harry Potter/Draco Malfoy onward. Rather than play innocent about these dynamics, “Sherlock” mines them heavily, for humor and frisson. Yet for all the “Wait, are they actually gay?” gags, the show is admirably committed to something more serious: the notion of Sherlock/Watson as both True Detective and True Romance. This is a real love affair, not a joke one.

It’s also a central shift from the original: Sherlock is still a detective, there are episodic mysteries, there is still Baker Street (now equipped with Web access), but the subject of the show is not so much Sherlock’s deductions as this relationship, which is itself a kind of mystery. Sherlock and Watson are best friends, certainly. They’re also chaste boyfriends, as well as a captain and his first mate. Mostly, though, they’re a god and a mortal, mutually besotted—the most impossible love affair of all. When viewers “read” this relationship (and Sherlock’s relationship with his other intra-show fans, who include the morgue employee, Molly, and Lestrade, the admiring police inspector), it’s similar to the way Sherlock “reads” a crime scene: intuiting clues that “normals” might dismiss. And, of course, Watson is implicitly a writer of Sherlock Holmes fan fiction. In Conan Doyle’s books, he keeps notes of their adventures, which he publishes in magazines. On the television show, naturally, it’s a blog.

The second episode dives into the question of what Watson is to Holmes, and it’s one of the best so far—a daring display of nested mysteries that also inverts the central love dynamic. Set mostly at Watson’s wedding, it’s framed by Holmes’s best-man toast, a dilatory, frequently belligerent, then suddenly tearjerking monologue, studded with anti-marriage zingers and hilarious accounts of their drunken bachelor party, in which the word clouds go blurry. (The signature of a good, serious TV drama is, ironically, a sense of humor: if it’s glum and gluey, run for it.)