When I was nineteen, I briefly shared a flat with a young lesbian couple who liked to go to parties dressed as Sherlock Holmes and Dr Watson. They accessorised with moustaches and dapper little hats and their own interpretation of what went on at Baker Street after Mrs Hudson had retired to bed. I had read and loved Arthur Conan Doyle’s stories as a kid. It was these girls, though, who taught me what fannishness could really mean - loving a story so hard, so shamelessly, that you let it change you, and maybe you change it in turn.

Some years later, I lived just off Baker Street, and walked to work past the Sherlock Holmes Hotel (complete with “Watson Burger” for American pilgrims) and a tube station plastered with tile profiles of a familiar figure with a deerstalker and pipe. The line between “fan fiction” and actual fiction has always been fuzzier than people wanted to admit - and the many worlds of Sherlock Holmes bear that out, none more so than the recent BBC "re-imagining", now in its third series. It is possible that no showrunners have ever been more gleefully fannish than Mark Gatiss and Steven Moffat.

You can almost hear the wrinkle-nosed whine in Guardian critic Mark Lawson’s voice when he describes the latest episodes of Sherlock as “blog-aware”. Lawson notes that “a running gag in the script [referenced] the wild and sometimes lurid online speculation in the real world about the circumstances in which the detective had apparently been able to fake his death in a rooftop fall at the end of the second series in 2012. Two lengthy sequences proved to be fan fantasies dreamed up by characters within the drama who were as obsessed with the fate of Sherlock as are viewers on the outside.” Lawson thinks this is a bad thing. Lawson, along with a great many other critics, would prefer that storytelling remained appropriately hierarchial - with writers and showrunners from the right backgrounds at the top, and everybody else watching along quietly and not making a fuss. I beg to differ, and here’s why.

Fan-ficcers are used to being treated as the pondscum of the nerd world, a few slimy feet below the table-top roleplayers and historical re-enacters. They don’t care, because they know - alright, alright, because we know - that fanfic is brilliant. I’ve spent many years hanging out in fanfic communities, mostly as a reader rather than a writer (my cringeworthy teenage Buffy slash was mostly done on paper). Fan fiction is where modern storytelling enters the realm of myth and folktale, where characters take on a life beyond the control of their authors, where they are let loose in communities with their own ideas about how to tell a story. More and more writers are coming out of those communities - not just E L James with her steamy Twilight rip-off, but fanficcers who break out into publishing their own original books, sometimes to great critical acclaim.

Excuse me, by the way, from taking a break from serious social justice writing to totally nerd out. It’s just that I’m desperately interested in stories, and who gets to tell them, and who has to listen.

Fan fiction is nothing new, and nor is the statement “fan fiction is nothing new”. Most discussions of the practice speak of Star Trek fanstories dating back to the sixties, and point to the influence of fan speculation on Joss Whedon when he was running Buffy. But actually, fan fiction is far older than that. It wasn’t until the Romantic period that originality was considered an essential skill for a storyteller to have. Before then, a truly great writer would be distinguished by his ability - and it usually was his ability - to provide a new reading of a classic tale or legend, to bring a familiar character or archetype viscerally to life.

Chaucer and Shakespeare lifted plots from everywhere. Poets in 17th century Britain got attention for their experimental poems by translating or rehashing great Greek and Latin works. Retelling old myths and creation legends, from fairytales to the Nordic folktales, is still a storytelling staple, especially given that Hermes and Odysseus are definitely characters in the public domain and there’s no risk of an bearded, betogaed ancient author hobbling into your office with a writ demanding a share of royalties.

Sherlock Holmes is now in the public domain, too. Just weeks ago, a Chicago judge ruled that any writer may use the characters of Holmes and Dr John Watson in whatever way they see fit. The ruling makes it clear that Sherlock Holmes has entered the territory of myth. The stories belong to everyone and noone. The same is tacitly true of Doctor Who, even though that story isn’t over yet: it’s a legend that extends across multiple platforms and spans decades, a saga that has gone on long enough that the people telling it today grew up with it as children, including the incoming lead actor, Peter Capaldi, whose adorable pubescent Fourth Doctor fan-art can be found online.

Martin Freeman as Dr Watson with Sherlock. Photo: BBC/Robert Viglasky

Stories like Harry Potter, Twilight and The Hunger Games occupy the same territory in the collective imagination, even though there is far less legal confusion over who owns the characters and how cross they would be if anyone were to plonk them into a new book or series for profit. For this reason, fan fiction is usually done for free. Even slash writers with a creative grasp of grammar learn to plaster disclaimers on their work.

Anyone can be involved in the story like this, though it might not always be a comfortable experience for the individual author. Last week, I had tea with a young fantasy novelist who confessed to me that, whilst she was pleased that her book was getting attention on Tumblr, reading fanfic about characters she had created unnerved her - not just because of the sense that someone was stealing them, but because she was worried it might overinfluence the books she has still to write.

Sherlock, however, doesn’t just engage with fan fiction - it is fan fiction. The writers most involved in Doctor Who and Sherlock are lifelong fans of both stories. Steven Moffat, who is showrunner on both series, would be the luckiest fanboy in the world if it wasn't for Mark Gatiss, who is co-creator and gets to play a plum lead role on Sherlock, as Mycroft Holmes, despite bearing absolutely no resemblance to Benedict Cumberbatch, whose brother he is supposed to be. It is as joyfully indulgent as any piece of fanfic I have ever encountered. That doesn’t make it critically redundant. Actually, it’s by far my favourite aspect of the show.

I’ve got a lot of problems with Steven Moffat’s narrative choices - starting with his ability to reduce even the most interesting female characters to bland archetypes - but fanservice isn’t one of them. Why shouldn’t a show engage with its fanbase? Especially this one, which is part of a suspicious explosion of recent Sherlockian homage - including Elementary, the camptastic Robert Downey Junior films and the new books that kicked off the Chicago lawsuit. The Holmes stories are now folklore, and that changes the implications of who’s in control.

The Great Detective has always had uppity fans. When Arthur Conan Doyle sent Holmes tumbling over the Reichenbach Falls in The Final Problem (1893), he was never intending to bring him back. Ten years later, he caved into public pressure and reanimated Holmes. Conan Doyle clearly envisioned Holmes as essentially fallible, a mortal man like any other; his reading public thought different, insisting that the detective could and should find a way to cheat death.

What Doctor Who and Sherlock offer us right now is a chance to see what modern fan fiction would look like if it was written by well-paid, well-respected middle-aged men with a big fat budget. That sort of fanfiction is usually referred to simply as “fiction”. Moffatt recently declared that Sherlock Holmes stories are fundamentally about “celebrating a clever man”. Are they? I thought they were about the triumph of reason over superstition, the power of friendship, and derring-do. I have no more right to an opinion on this than Moffatt does - but no less, and nor does anyone else who loves the Sherlock stories, including whoever came up with the “Watson Burger”.

And that’s where the problem is. Moffat and Gatiss may write with one eye on their fanbase, but their ideal fanbase still looks a lot like them, which is what people of their demographic usually mean when they talk about writing for a “mainstream audience”. They write the sort of stories that would interest smart teenage boys who grew up in the 1970s and 1960s; stories about “clever men” in which women are dispensable love objects, figures of derision, or both. The pining, put-upon character of Molly Hooper in Sherlock, one of the few characters with no easy equivalent in the original stories, is painful to watch as she mopes around after Holmes like a lovesick puppy. A lot of fanfic sets out to put that right, ensuring that Molly gets the boy, her own adventure, revenge, or all three.

The idea persists that one person’s wankfest is worthier than another. During a preview panel event at the British Film Institute some weeks ago, writer and Times critic Caitlin Moran got Sherlock actors Benedict Cumberbatch and Martin Freeman to read out some steamy fanfic she had found on the internet for the amusement of the crowd*. The author of the piece was upset, accusing Moran of “humiliating me, taking my writing out of context without permission, belittling it and using it to embarrass actors who I deeply admire”. But what was read out can hardly have been sillier than the endless nudge-nudge implications about Sherlock and Watson by Moffatt’s modern-day Mrs Hudson. This series has devoted serious time, including all of last week’s episode, to the question that has fascinated fans from the start: how much, and in what way, are the doctor and the detective really involved?

The dream-scenario in which Sherlock and Moriarty are actually lovers playing a trick on Watson and end up in a crafty rooftop clinch was so much fun that the people sitting either side of me as it aired had to hold me down lest I levitate off the sofa. The scene then cuts to a grumpy teenage member of a conspiracy club - a lovely cameo by Sharon Rooney - explaining that the “Sherlock elopes with his nemesis” story is hardly less fanciful than anything anyone else has come up with. She’s dead right. Later in the series two men are stabbed with skewers but we’re supposed to believe they can’t feel it because their uniforms are too tight, and a train full of explosives at an abandoned underground station right under parliament somehow remains undetected until zero hour. All of these things are significantly less plausible than gay sex. I’m just saying.

What is significant about unofficial, extra-canonical fan fiction is that it often spins the kind of stories that showrunners wouldn’t think to tell, because fanficcers often come from a different demographic. The discomfort seems to be not that the shows are being reinterpreted by fans, but that they are being reinterpreted by the wrong sorts of fans - women, people of colour, queer kids, horny teenagers, people who are not professional writers, people who actually care about continuity (sorry). The proper way for cultural mythmaking to progress, it is implied, is for privileged men to recreate the works of privileged men from previous generations whilst everyone else listens quietly. That’s how it’s always been done. That’s how it should be done in the future, whatever Tumblr says.

But time can be rewritten. Myths can bend and change. Something new and exciting is happening in the world of storytelling, and fans are an important part of it. All kinds of fans, from obsessive cupboard-dwellers to the shouty social justice crowd to livejournal perverts who just want to know what Sherlock would have to say about the chemical composition of personal lubricant.

On this and other matters, it doesn’t do showrunners any harm to pay attention to their fans. We are living in a world of stories where thousands of new voices from diverse communities are speaking up, sharing ideas and creating new worlds out of the shadows of the ones we knew as children - but so far, a handful of professional chaps still get to make the decisions. Now, where have we heard that one before?

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*I understand the writer’s distress, but I don’t believe Moran intended to mock the posture of fannishness. A lot of her own job involves squealing in an entertaining, relatable fashion about shows and books she loves. Exhibit A: her entire public response to the potential casting of Ben Whishaw as The Doctor was “NOT ENOUGH WANKING IN THE WORLD.”