'Fifty Shades of Grey' is the latest literary phenomenon to storm the world - and with a mega-budget movie mere months away, and speculation already rife over which A-List actors who will play the lead roles, interest shows no signs of abating.
What sets it apart from 'Harry Potter', 'The Lord of the Rings' and countless other book-to-movie adaptations however is that it's the first franchise to be based on another.
For while 'Fifty Shades of Grey' is a romance of the kinkiest order, it originally started life as 'Twilight' fan fiction, written by overly-amorous fangirl E.L. James.
But sexing up do-gooders Bella and Edward is just the tip of the weird, bizarre and often terrifying world of fan fiction based on popular movies that lurks in the bowels of the internet. Read on at your peril.
Fan fiction comes in many colourful shapes and forms. At its most innocent there's warm and fuzzy tales of happier alternate endings. One of our favourites is 'what would've happened if Titanic's Jack and Rose had both fit on that driftwood?'. Spread over 25 chapters, it sees the duo have kids, grow old and Jack even reunites from Fabrizio from the movie.
There's also the odd tale bizarre alternate universes, such as one story that asks the pertinent question: 'what if Twilight's Edward Cullen were a baker?'
Let's be honest though, most fan fiction is filth - weird erotic fiction that sees some surprising characters pair up. These were far too rude for us to link to (sorry!).
Ever dreamt of an steamy romance that takes 'Fast & Furious' bromance to the next level? Do you just want to read up on 'The Lord of the Rings' Arwen and Eowyn getting it on? Or, if you really want to embrace the weird, why not check out the raunchy exploits of some 'Jurassic Park' workers being sexually attacked by a velociraptor?
But just when you think a Harry Potter 'Harry/Hermione/Dobby' hook-up is the weirdest place your brain could possibly go, then I'd like to introduce you to Mpreg - which stands for 'male pregnancy'.
While the stories themselves are focused more on the characters caring and nurturing for each other than horrific descriptions of the birthing process, once someone puts the idea of a pregnant Frodo/doting father-to-be Aragorn relationship into your head you're unlikely to watch 'The Fellowship of the Ring' in the same way ever again.
Amusingly enough, none of these fall into the arguably craziest sub-genre of them all - 'crack fiction' - defined to make no real sense at all.
Case in point - a story set in 'The Dark Knight' universe in which a drunk Batman tells Commissioner Gordon how much he loves cookies and his iPhone, or a tale where the cast of 'The Land Before Time' encounter the crew of a Soviet submarine.
Which brings us onto the most hilarious and baffling sub-genre of all - the crossover. While the majority are yet again lewd (good luck getting through the disturbing sexual adventures of Harry Potter's Bill Weasley and Indiana Jones), the more sedate stories are often genius. The idea of a 'Twilight' and 'Avatar' team-up or a 'The Lion King' and 'Avengers' crossover are almost insane enough to work. Almost.
Finally, there's fan fiction dedicated to (intentionally) making you laugh. And while the quality of the writing is often more hilarious than the subject matter itself, it's hard to disagree with a short story that brings the worlds of 'Saw' and 'High School Musical' together to devastating effect.
Words: Matt Risely