Think of Douglas Adams and you'll think of The Hitchhiker's Guide To The Galaxy, but what of his self-proclaimed holistic detective? Dirk Gently is often eclipsed by the success of Hitchhiker, and yet, Adams' two Dirk books are some of his finest literary works, which inspired radio plays, stage plays, comics and two entirely different TV series. If you're new to the slightly confusing world of Dirk, now is definitely the time to explore his novels and many spin-offs.

The first book, titled simply Dirk Gently's Holistic Detective Agency was published in 1987. The plot was born out of Douglas's time working as Doctor Who script editor, almost ten years prior, in 1979. For his series (the seventeenth, the penultimate to star Tom Baker as the Doctor) he wrote two serials, the first was called City of Death and the second was the notorious Shada (considered notorious, as it was never completed owing to a strike). Both these stories contained plot elements which became key to the first Dirk novel. And, in a way, this fact has hindered the success of Adams' detective ever since, with many assuming the book to be a bit of a Doctor Who offshoot.

In fact, apart from a few salient plot points, Dirk Gently is written in a style that is very different to both Doctor Who and The Hitchhiker's Guide To The Galaxy. Taking the genre of urban fantasy to new heights, the book ripped up the rule book and pulled a narrative trick so impressive that it could give Agatha Christie's The Murder of Roger Ackroyd a run for its money.

We first meet Dirk through the eyes of Richard McDuff, a former college student, revisiting Cambridge University to meet his old Professor for a reunion dinner. When his former tutor, Professor Chronotis, starts mentioning Dirk, Richard is surprised he hadn't seen, or indeed heard, of his old friend in years, but then, can Dirk Gently ever be truly described as a 'friend'? Adams describes this as "not so much unlikely, more a sort of mismatching of concepts, like the idea of the Suez Crisis popping out for a bun".

For all the world, Dirk Gently is a mysterious recluse, who left university disgraced in the back of a police car. It is only when his name resurfaces that Richard's life starts getting decidedly weird. And when you throw in the murder of his employer and potential brother-in-law, Gordon Way, things become very strange indeed.

The plot of Dirk Gently's Holistic Detective Agency is so notoriously complicated that even Douglas Adams himself revealed that when re-visiting the novel years later, even he didn't understand it. But the extraordinary thing about the book is that it's so good that this doesn't seem to matter.

Douglas's writing talent is world renown, but here he seems to be at his creative peak. Each sentence reads as if it were a line in a poem. The book is effortlessly funny, but at the same time, it is darker than Adams' earlier work. Arguably his greatest ever character arc defines this book; it features Richard's employer, the head of a moderately successful software company, Gordon Way, who is brutally gunned down and subsequently becomes a ghost. The overzealous boss full of late eighties decadence isn't particularly likeable, but you can't hate him either, and can only feel sympathy as he grapples with his miserable fate.

The book deals with all these big issues with a light touch, throwing in musings on religion, the beauty of classical music and the harsh reality of death in an off-hand way, and that's just scratching the surface of the novel's contents. Dirk's search for missing cats, a horse in a bathroom and an Electric Monk all wrap into a bootstrap time travel paradox that will continue leaving readers scratching their heads for years to come.

The second novel, The Long Dark Tea-Time of the Soul, introduced us to Douglas's first female protagonist, Kate Schechter. She is a confident American journalist whose no-nonsense personality clashes with Dirk Gently's enigmatic nature, not that either character has much to do with each other.

Kate is preoccupied with Thor the Thunder God, who has apparently taken a liking to her, whilst Dirk is racked with guilt over the murder of his potential client, Geoffrey Anstey, whom he had very unwisely given the brush off. With Norse mythology, tired Gods and tired people at the forefront, this is a novel travelling at a much less frenetic pace than its predecessor and it is the origin of one of Douglas's greatest quotes:

"I may not have gone where I intended to go, but I think I have ended up where I needed to be."

As the story builds to its climax, it is Dirk's quest for a cigarette that leads him to King's Cross Station. He witnesses the poverty and despair of the homeless people that line the station and the surrounding streets, and, as many of them start abruptly disappearing in the heart of Euston's great, majestic and (as it was then) abandoned hotel and into the realm of the Gods, Dirk feels compelled to follow them.

Douglas Adams designed both Dirk books to be dense, rich, and to give only glimpses of what's really happening, which demand repeat reading. So, how do you go about adapting these complex stories into any other medium? Well, the first people to try it were James Goss and Arvind Ethan David. Two students who became so enchanted by the first Dirk Gently book that they both set about adapting the novel into a play. How was this possible? They explained:

"We decide, with the careless hubris of teenagers, to adapt a novel. Not just any novel of course, but Dirk Gently's Holistic Detective Agency, a novel with a ludicrously complicated plot which spans 4 billion years, at least three alternative universes; a cast comprising vampire-detectives, aliens, ghosts, and an electric monk; all linked by the central theme of 'the fundamental interconnectedness of all things' (hence 'holistic'). We were sixteen, so didn't bother to get rights clearance but just staged the thing. It was very short (about an hour) and by all accounts, amusing but utterly incomprehensible. It had a budget of forty pounds. A video recording of this production exists, but no-one is sure exactly why."

The play continued to be developed and as it moved into bigger theatres around Oxford it attracted some serious praise. Douglas Adams himself loved it, the writers affectionally remembered him coming to see the production on more than one occasion. It was even said to have inspired Adams to work on a movie script for Dirk Gently (although a copy of this has not been discovered).

As referenced, a recording of the play exists, but sadly it isn't in the public domain. However, the script book is, and it is a must have for any fan who wants to experience an alternative to that very baffling ending of the original book. Yet, this wasn't the last involvement Arvind Ethan David would have with the character of Dirk Gently, as in 2016 he became one of the executive producers of BBC America's Dirk Gently's Holistic Detective Agency (which was created by Max Landis).

In this series, Dirk Gently was a far more optimistic, youthful and all-round excitable individual than his world-weary book counterpart. He was played by Samuel Barnett and Richard (renamed as Todd Brotzman) was played by Elijah Wood. Together they became a sort of Doctor Who style duo, solving supernatural mysteries using Dirk's belief in the fundamental interconnectedness of all things. The series had an expensive and grandiose feel to rival The Hitchhiker's Guide To The Galaxy's 2005 film. It was an all-out American TV series and at its core, it was very different to the books. The show developed a serious cult following during its two series run, and with an entirely new set of characters and its own lore, it was perhaps slightly ahead of the curb when it came to the sci-fi fantasy TV series boom. Fans were left bereft when the show was cancelled in 2017 (although the adventures continued in comic book form).

The BBC America series is the most famous adaptation of Dirk Gently as a concept. However, most Hitchhiker fans would probably consider the two Radio 4 adaptations of the books as the ultimate spiritual successors to the Hitchhikers. Afterall, these were made in the wake of the success of (legendary radio producer) Dirk Maggs' continuation of The Hitchhiker's Guide To The Galaxy radio series (from 2003 - 2018).

Both series of Dirk Gently made for Radio 4 featured an all-star cast with Harry Enfield at the helm as the titular detective. Enfield's casting was faithful to the man Douglas had apparently based the character on, his friend and literary agent, Michael Bywater, who even acted out a scene as Dirk for The South Bank Show focused on Adams.

The short sketch was the only on-screen adaptation of Dirk Gently made in Douglas's lifetime, and it's fascinating to compare Bywater's performance to Enfield's. His casting was perhaps the closest to Adams' original vision of the holistic detective and the supporting cast feel similarly apposite. Billy Boyd puts just enough naive charm into Richard and Jim Carter, in particular, makes an astonishingly great Gilks across both plays. The real stand out though is Robert Duncan as Gordon Way, who delivers in all aspects of this incredibly challenging part. Whilst Olivia Colman puts in a wonderful comic performance in an elevated role for Dirk's secretary, Janice Pierce.

In the sequel, The Long Dark Teatime of the Soul, Laurel Lefkow starred as Kate. She had previously worked with Douglas Adams during the development of his last project (that he saw to its competition) the video game, Starship Titanic, where she had voiced Titania. Another friend and former Dr Who, Peter Davison, starred as the villainous Simon Draycott and a voice very familiar to Hitchhiker fans, Stephen Moore (the voice of everybody's favourite depressed robot, Marvin) played Odin.

That every former friend and colleague wanted to rally round and make something special for him, proved what a remarkable man Douglas Adams truly was. Dirk Maggs had achieved something very impressive with these two Radio 4 series, and was intending to create a story out of the fragments of a Dirk novel published in The Salmon of Doubt (a collection of Adams' writings published posthumously) but, unfortunately, this never came to fruition.

There was, of course, one more adaptation, the 2010 BBC Four TV series starring Stephen Mangan as Dirk and Darren Boyd as Richard. Much like the BBC America series, this Dirk Gently adaptation also took the idea of Dirk and Richard as a double act, solving sci-fi themed mysteries with a comic twist. The sitcom-like show took on its own unique flavour whilst still featuring a few key characters, such as Jason Watkins' brilliant turn as Gilks. The pilot episode is noted for a stand-out performance by the late great Doreen Mantle, a performance no One Foot In The Grave fan will want to miss. But, as the series was tucked away on BBC Four, this version of Dirk's universe wasn't seen by many, and it ended after just one series. Again, the idea was ahead of its time, and if today it were broadcast as a BBC Two comedy, the series would be a sure-fire hit.

Some might say that Dirk Gently is due another remake, but in the meantime, there is plenty to choose from if you want to get involved in the weird world of Douglas Adams' Holistic detective.