I was still at school when the first Terry Pratchett Discworld story appeared. My friends and I were all fans of fantasy and science fiction, whether it be video games, movies, trading cards, table-top RPGs or, of course, books. To us, these were no stranger things, yet this odd little novel, bracketed within a colourfully chaotic wraparound cover instantly became a favourite in the classroom. It was called The Colour Of Magic, and it began the saga of incompetent wizard, Rincewind, resident of Ankh-Morpork, the largest city upon the Discworld, a round circle astride four elephants and the Great A'Tuin, a giant sea turtle. Yes, it still sounds bonkers, and despite the gentle ribbing (ok, not always so gentle) of our favoured genres, The Colour Of Magic became an instant hit.

As a ZX Spectrum owner, I naturally made a beeline for the 1986 Piranha/Delta 4 graphic text adventure based on this book. But further episodes were not forthcoming, and I had to wait until 1995's DOS, PlayStation and Saturn Discworld game to scratch that Ankh-Morpork itch once more. Discworld 2: Missing Presumed...!? followed a year later, but it wasn't until 1999 and Perfect Entertainment's third Pratchett adventure that interactive Pratchett finally clicked for me. Maybe it's the combination of the author's vivid world and a cinematic genre I love, a bizarre amalgamation that proves a fertile bed for comedy; or maybe it's because everyone involved was at the peak of their powers, at one with both material and technology.

Inside the Café Ankh, an establishment clearly modelled on Rick's Café from Casablanca.

One such participant was Chris Bateman, lead writer and designer on Discworld Noir. "That was an astonishing opportunity to give to someone so early in their career!" he exclaims, before adding a surprising admission. "But I confess that I had only read The Colour Of Magic when I joined Perfect, and I was a bit disappointed by it. I loved the first part, inspired by Fafhrd and the Grey Mouser, but otherwise it seemed a little random." However, having begun work on Discworld 2 shortly after joining the developer, Bateman soon fell in love with the series. "I know a lot of fans love the Rincewind stories the best, but the City Watch for me is the heart and soul of Terry's world." Unsurprisingly, this band of dishevelled urban guardians would go on to have a key part to play in the next game, Discworld Noir.

As with the previous Discworld games, development was overseen by industry veteran Gregg Barnett, and it was Barnett who pitched the idea to the author after the pair mutually agreed that the Rincewind stories had run their course. "Terry was thrilled at the concept of a hardboiled detective in Ankh-Morpork," remembers Bateman, "and the plotting began with me coming up with a synopsis based around the assassin character, Teppic, from Pyramids. But Gregg shot that down in flames and sent me off to read Raymond Chandler and Dashiell Hammett."

Having consumed every relevant book he could get his hands on, Bateman begun watching their cinematic adaptations as well. "It ended up becoming a Humphrey Bogart and Lauren Bacall marathon - not exactly the toughest research job I've ever had to do! And that's how you get nods to movies such as Casablanca, because movies like The Big Sleep and To Have And Have Not had Bogey and Bacall in, so it made sense to work through them all."

But perhaps most memorably for Discworld Noir fans, it's not the Chandler/Hammett influence that most lingers, but that of another, altogether different author. "There are loads of sideways nods to HP Lovecraft in Terry's books," notes Bateman, "and in particular, in Moving Pictures. So it wasn't like I was adding something that wasn't already there, I just picked up everything dark that Terry had already put to work in the Discworld, and made good use of it for my own story." The main thrust of Lovecraftian (and indeed classic) horror is the lycanthropic transformation that its protagonist suffers from act three onwards. "I think I must have floated the idea of a werewolf plotline with Gregg, and somehow he and Terry let me get away with it," laughs Bateman. As the hirsute beast, detective Lewton is able to divine fresh clues utilising his hyper-sensitive nose and an accompanying scent inventory. Explains Bateman, "I was looking for ways to use object inventories, which were a staple element of adventure games, in new and unusual ways. And smelling your way through crime solving is certainly unusual!"

Discworld Noir begins, appropriately, with a femme fatale appearing in the detective's office.

Unlike the previous Discworld games, Discworld Noir features totally new and original material, mixed in with themes and characters from the novels such as Commander Vimes and the City Watch. In the finest tradition of film noir, the hero begins the story at its end, reciting the details having met his untimely demise at the hands of an unknown attacker. The labyrinthine tale involves a series of ritualistic murders, for which Lewton himself is initially accused of, and a beautiful femme fatale named Carlotta, who asks Lewton to track someone down, the whole thing wrapped up into a point and click adventure. In 1999. "Yes, we were quietly terrified about the market giving way under our feet, especially near the end," winces Bateman. "Games like Tomb Raider and Resident Evil had hit in 1996, and there was a definite sense that the writing was on the wall for adventure games." The team's ambition also displayed in the game's engine, a significant upgrade from the sprite-based Tinsel engine of the previous Discworld games. "We needed a whole new system to demonstrate the world in 3D polygons," explains Bateman. "Part of it was the same code that we'd used before, but a lot of hard work was done in the transition [from 2D to 3D]."

Plot, characters and graphics is one thing. Or rather three. But the inclusion of humour was the primary focus of Discworld Noir. Having read two dozen of Pratchett's books at the start of development, Bateman felt he had a grasp on the way the humour worked, at least broadly. Fortunately, the one man who knew the way that Discworld ticked far more than anyone else modestly offered his services to Perfect Entertainment. "I met Terry on several occasions," beams Bateman, "and while he didn't come to the offices much, he always came to the launch parties, and I got to know him and his agent, Colin Smythe, quite well. By far the most time I spent with him was during the promotion of the game, as they put the press in front of Terry, and there was me, lurking in the background as a complete unknown!" In practical terms, Pratchett made an editing pass on the entire 12,000 line script of Discworld Noir, fixing up anything he felt was off kilter. "He didn't change much," adds Bateman proudly, "but every change he made was golden."

Yet despite its convoluted storyline, mournful music, comical asides and exquisite graphics, there are two facets of Discworld Noir that stand out for me today, and hopefully for other fans of the game too. "Oh the voice cast, that was a lot of fun!" grins Bateman. How about this for a roster of talent: Rob Brydon, Robert Llewellyn, Nigel Planer, Kate Robbins. "Kate was just phenomenal, and she came in on the day, having already read the sheet music for our musical number, and delivered it one take - amazing!" But with comedian Rob Brydon handling the bulk of the work, a multitude of hard-boiled monologues, and Planer impressing Bateman with his work ethic ("It took him a while to get the booth how he wanted...then he rolled through his lines with hardly any need for retakes"), it was left to Red Dwarf actor Robert Llewellyn, at the time commencing a successful writing career, to provide the levity. "I had the most fun with Robert, I'd love to work with him again," admits the designer. "Throughout the recording sessions, I was quietly keeping score of whose jokes were getting the most laughs, mine or Terry's. Robert was far and away the one cracking up the most at both our jokes." With voice talent fundamental in a story-driven game such as Discworld Noir, Bateman has the magnanimity to admit that its stellar cast transformed his script into something far more than it could ever have been otherwise.

The wonderful notebook, assisting Lewton in his enquiries.

For all that, it's with Discworld Noir's notebook that lies its greatest mechanic and noir sensibility. In a game where discovering clues, discerning their relevance and acting upon them is key, this was something that Bateman and his colleagues felt they absolutely had to nail. "The moment it was confirmed to be a detective story I knew that we couldn't just rely on just an object inventory," he notes. "Adventure games up to that point had mostly either had conversation topics that were isolated from everything else or objects that you could talk to people about." To Bateman, it seemed obvious that they should combine the inventory and dialogue engine into one system, the notebook.

This series of pages seeks to help the player combine their thoughts, and make deductions based on the entries held within. The result is a tremendous (and realistic) device that manages to actually make the player feel like a real detective, quizzing suspects on both relevant and seemingly irrelevant subjects, narrowing down the field of enquiry as any good gumshoe would. "It involved quite a bit of graft from me and one of the programmers, Mark Judge," adds Bateman. "I designed a scripting language that organised the dialogue into topics, each of which was an object inside the engine; and then I specified scripts for each character that gave them responses to every topic that was relevant for them."

There were two more concepts that worked invisibly within the notebook to help the player. Firstly, a useful error system is employed that causes interrogatees to helpfully guide the player in the right direction of enquiry, rather than respond with a standard rebuttal; and secondly, there's the concept of hyperclues. "That was inspired by my experiences of the new-fangled internet in the early Nineties. To the player, clues appeared as text in the notebook, and were collected onto pages with a common theme. But also, some clues appeared on multiple pages, and those were hyperclues which you could double-click on to go to the other page the clue was on. It was terribly useful, but I'm not sure every player discovered it!"

20 years ago this year, the Windows version of this forgotten classic was released to praiseworthy reviews and a handful of adventure-of-the-year awards. But sales were poor. "The point and click genre had had its day by 1999," recounts Bateman sadly. "All the big publishers were foaming at the mouth about first-person shooter games and other high-octane polygonal fancies. If we'd had more marketing support, we could certainly have sold more...but how much more is hard to say."

The situation was not helped by background strife at both Perfect and publisher GT Interactive Europe. With the former involved in a costly dispute with Psygnosis, the latter's woes appeared largely self-inflicted. "GT Europe were insane," tells Bateman. "They were given a huge war chest by the parent company, but they spent it extravagantly, in ways that were not sensible. At the trade show ECTS in London, for instance, they made a perfect duplicate of their offices inside Kensington Olympia. But, of course, nobody except GT employees had ever been to their offices, so it was a complete grand folly."

Throughout the many conversations, Lewton can begin to decipher the clues he discovers.

Yet Discworld Noir remains an outstanding game, chock full of ideas and atmosphere, that didn't so much as slip under the radar as barely take off in the first place. While some commented cruelly on the foolhardiness of releasing a point and click adventure in the late 90s, there is no doubting the ambition and bravery behind it. "There's a thin line between brave and foolhardy!" snorts Bateman. "If anyone on the project was courageous, it was Gregg and Angela [Sutherland] for taking such a chance on a young game designer with wild ideas and no experience. I still can't believe how fortunate I was - I had Terry Pratchett as script editor and a voice cast the envy of anyone else in games at the time."

If, like me, you remember Discworld Noir with unalloyed fondness, you may be interested in a - ahem - bribe that co-creator Chris Bateman has in mind. His latest game, Silk, while sharing little technically with Discworld Noir, save its concept of a bespoke machine-processed scripting language, involves puzzle-solving of an altogether different kind. "I've worked on more than 50 games, but three in particular have a special place in my heart," he reveals. "Discworld Noir, Ghost Master and Silk, with Silk being the underdog because it had to fight every step of the way just for to exist. And while we're talking Silk, let me say this: Noir fans should support it every way they can, because it's the only way they'll get a Discworld Noir spiritual successor." Chris's latest design, Requia Noir, acutely evinces the themes that make Discworld Noir such a fantastic experience. "Call it bribery if you like," he smiles. "But I prefer to think of it as a creative deployment of persuasion." Now that sounds like something even the City Watch would approve of.

Dedicated to the memory of Sir Terry Pratchett, OBE, still missed dearly four years since, and still blamed by my old form tutor for disrupting her class.