As we all know, television detectives come with all manner of USPs – their methodologies ranging from forensics (the CSI franchise), deduction (Sherlock etc) and the supernatural (Medium) to the powers of observation (The Mentalist) and the ability to spot a fib at 40 paces (Lie to Me). There's even one who suffers from a multiple personality disorder (Shattered). But what if a crime writer were to approach a TV executive with the idea of a sleuth who solved cases using the principles of quantum mechanics?
Well, it's a novel angle, but the commissioner would think he was dealing with either a mad man or a genius – and in the case of Dirk Gently, the assessment wouldn't be far off the mark. For Dirk Gently, with his belief in "the fundamental inter-connectedness of all things", was a creation of the late Douglas Adams, a genius in many people's eyes.
Adams is of course better known as the author who dragged science fiction into the Monty Python era with The Hitchhiker's Guide to the Galaxy. And if any detective was going to not so much break the genre mould as cheerfully throw it out of the window, it was going to be the owner of Dirk Gently's Holistic Detective Agency. That's the title of Adams's 1987 fantasy detective novel, which is described on its cover as a "thumping good detective-ghost-horror-whodunit-time travel-romantic-musical-comedy-epic" – the sort of Pythonesque flourish (Adams was a close friend and collaborator of Graham Chapman and occasional contributor to Monty Python) cherished by his fan club. But will they cherish a new TV adaptation of the book, especially since most of the plot has been jettisoned, along with over half the title?
"The books as they are written, you cannot adapt them for the screen," says Stephen Mangan, the star of BBC4's Dirk Gently. "There's just far too much in them... too many ideas, too many plots and storylines, especially if you making a one-hour drama."
And yet Adams himself, who died of a heart attack in 2001 at the early age of 49, believed that the two Dirk Gently novels (The Long Dark Tea-Time of the Soul is the sequel) would be easier to film than The Hitchhiker's Guide to the Galaxy. But then he wasn't envisaging a BBC4 budget, and the job of squeezing his infinite imagination into a very finite 60 minutes of low-budget TV drama. That fell to Howard Overman, creator of E4's Asbo-teens fantasy Misfits.
"The BBC wanted a returning detective show, one doable on a BBC4 budget," says Overman. "The book involves all sorts of crazy ideas, characters wandering around different planets, rainforests, unicorns and God knows what else. So what we did was to take the character and come up with a new story."
And that he has. Co-starring Helen Baxendale and Darren Boyd, his story begins with Gently (Mangan) being hired by a little old lady to find her lost cat. All things being connected ("an obese American on the other side of the Atlantic fails to pay his mortgage and you're unemployed", is how Gently gives his methodology a contemporary spin), the search for the missing moggie takes in a stolen laptop, time travel, a double-murder and a legendary experiment in quantum mechanics known as Schröndinger's Cat. "There's no other detective who solves cases with such a unique way," says Overman. "So you start with a missing cat and then you can go anywhere you like and then draw them together in a classic Agatha Christie-style denouement."
With his unruly mop of hair and elbow patches, and driving an Austin Princess, Overman's Dirk is also a classic English eccentric – a recognisably shabby relation of Doctor Who. "Adams actually wrote for Doctor Who in the Seventies and these books were slightly inspired by a couple of scripts that didn't get filmed," says Mangan. "So there is definitely a bit of that in it somewhere."
'Dirk Gently' is on tomorrow at 9pm on BBC4