We'd love another series of this double act of Stephen Mangan as Douglas Adams's eccentric holistic detective and Darren Boyd as his eyebrow-raising assistant. But for now, we'll just have to savour this twisting and turning final episode, in which Dirk is framed after several of his clients are murdered.

Not enjoying the Dirk Gently pilot very much, I predicted this "comedy-drama" - another warning required for these - wouldn't get a series. So here it is. I like Stephen Mangan, who plays the holistic private eye; I like Darren Boyd. But the most telling line was the latter's "Wow, that's... bollocks."

Following on from a successful pilot in late 2010, BBC Four's commissioned a full series of this comedy drama loosely based on the novels by Douglas Adams, and starring Stephen Mangan as the holistic detective.

The first episode in the series, which sees Dirk deal with a murder that has links to the Pentagon, contains some funny situations created by Howard Overman, the man behind the adaptation. Such things include Dirk breaking into a house of the murder victim by smashing a glass door being witnessed by those inside. Then there's Dirk surveillance operation which goes completely wrong thanks to his partner/assistant MacDuff's (Darren Boyd) new chair.

However, personally speaking I'm one of those people who would have been happier with the original stories being adapted for the screen rather than having new ones developed. While it does contain some elements from the original books, such as Zen navigation (instead of using a map to go where you want to go, you follow someone who looks like they know where they go, often leading you to somewhere you need to be), it would be nice to see Adams's original tales on screen.

Still, if you too are annoyed by the lack of faithfulness in this adaptation, there are always the more faithful Radio 4 stories starring Harry Enfield, which does follow Adam's work much more closely (Electric Monks and Norse Gods included).

In Stephen Mangan's own words, Dirk Gently is "charming, ­irritating, bright, funny, hapless, unreadable, transparent, roguish, chaotic, philanthropic and possibly dishonest".

That's a lot of character traits to be dealing with, but we discover yet another, equally surprising side of his personality tonight as he shares fish and chips with a new female friend.

Dirk and MacDuff (Darren Boyd) are at Dirk's old college at Cambridge to take up the post of head of security.

His former teacher, Professor Jericho (Bill Paterson), is trying to develop artificial ­intelligence and he's afraid that someone is attempting to steal his research.

However, Dirk's more concerned with breaking into the college records to find out why he was expelled as a student.

It's just a shame creator Douglas Adams isn't around to see how Howard Overman has ­transferred Dirk to the screen.

He'd definitely approve.

The main plus point for Dirk Gently is having the consistently great Stephen Mangan on board in the lead role - he's certainly one of our most watchable comedy actors, and is in particularly fine form as the infuriatingly self-sure (but still rather lovely) private detective who believes in 'the fundamental interconnectedness of all things'.

Since the superb Sky One comedy Spy, my eyes have also been belatedly opened to the huge talent of Darren Boyd, who plays Gently's rather more conventional assistant-slash-business-partner MacDuff - so all in all I can't help but come to Dirk Gently with a whole heap of goodwill.

But I think my enjoyment of this episode can be put down to more than that. It's a great-looking thing, and the script was sharper than the pilot - I particularly enjoyed the line "his cheque bounced like the proverbial basketball... on a trampoline." Miss out the word "proverbial" there and it's prosaic; with it, it's a winner. There were little gems like this throughout the hour, and Douglas Adams's genius sense of the absurd is perfectly encapsulated in the idea of 'zen navigation': find a car that looks like it knows where it's going, and follow it. Pretty silly, but highly entertaining.

I struggled with Dirk Gently (Monday, BBC Four). It had nothing to do with Stephen Mangan's considerable comedic talents, still less with Darren Boyd who plays Macduff, the Dr Watson to Dirk's Holmes. It is more to do with my devotion to Douglas Adams, upon whose comic novel this series is based. Adams was never well served by TV or film adaptations of his work, even big budget ones such as the 2005 film of The Hitchhiker's Guide to the Galaxy. His books always worked much better as radio adaptations that could leave the listener's imagination to fill in the gaps (indeed the Radio 4 version of Hitchhiker's even managed to be better than the book).

Jeeves and Wooster was similarly hard to make work on screen. Though Fry and Laurie's version was as good as any TV adaptation could be, it tried to tell the story through dialogue alone, which merely drew attention to the silliness of the plots. In PG Wodehouse, as in Douglas Adams, 90 per cent of the pleasure is in the prose, the narration, the felicities of language.

Over the course of a novel, Adams could afford to be quite subtle about Dirk's big idea, that all things are fundamentally interconnected. A TV adaptation can't be, and, as it keeps labouring the point, you find yourself saying: "Yes, yes, I get it." Perhaps as the series develops they will tone down this side of things.

Finally, and this is an anoraky point, Mangan looks nothing like the Dirk of the novels. At Cambridge Dirk was "rounder than the average undergraduate and wore more hats", and in later life he becomes rounder still, and scruffier, and more chaotic. Mangan seems too neat, too thin, too orderly.

With regards to Dirk Gently, there's nothing wrong with the actors Stephen Mangan and Darren Boyd and some nice moments from Howard Overman's script. It's just that those qualities in the end spread a little too thinly over a nonsensical thriller plot.

It's supposed to be nonsensical, of course - Dirk's belief that "everything is interconnected" pretty much necessitating a chain of wildly improbable coincidences and consequences. But since anything can happen you don't very much care about anything that does, and Dirk's metaphysical musings about "Zen navigation" and the complexity of the world begin to get repetitious quite quickly. There were laughs, including a nice reveal when Mangan opened a Valentine's card in the middle of a complacent speech about his powers of attraction to find that the inscription inside read "I hate you, you're a pig". But they were far too widely spaced in a script that could have done with a lot more editing. Scorning someone's belief in astrology, Dirk asked him whether he really believed that planets "billions of light years" away could affect human destiny. Millions of miles would cover it, Dirk, and yes, you might justly point out that this scientific pedantry is irrelevant. But I probably wouldn't have noticed if he hadn't used the same phrase three times. Or if I'd been laughing enough to distract myself.

Writer Douglas Adams had a breathtakingly fertile mind, and TV has sometimes proved a limited medium for capturing his flights of fancy. But Howard (Misfits) Overman has plucked the comic essence of Adams from his novel Dirk Gently's Holistic Detective Agency and worked it into a digestible, enjoyably eccentric format.

A well-received pilot in 2010 has spawned this three-part commission - a first for BBC4 and we hope not their last - starring Stephen Mangan as the shambolic, slightly annoying gumshoe and "Everywhere Man" Darren Boyd as his flinching sidekick.

The endearingly chaotic story about Pentagon surveillance, horoscopes and a toy pig is suitably Adamsian, and overall it makes for a quirky alternative to Sherlock - I think it can go quirkier still. But the show belongs to Boyd: without expending any effort, he has all the funniest moments.

Since 2010's largely successful pilot episode based on Douglas Adams's comic detective novels, public appreciation of bromantic bickering between sociopathic north London super-sleuths and their prissy but redoubtable sidemen has risen considerably. A fluffy pig, some bogus horoscopes and an extramarital affair are the dots that Stephen Mangan's 'holistic' private eye - a shaggy-haired Sherlock with a belief in the interconnectedness of seemingly random coincidences - and grounded associate Darren Boyd must join to stop some global meltdown or other. It ditches the more ornate lunacy and the dark, portentous undercurrents of the Adams originals, but just enough of his charm, fun and spirit of closeted anarchy remains to appeal to fans as well as newcomers.

Compared to Whitechapel over on ITV1, the cases ­investigated by Dirk Gently's Holistic Detective Agency make perfect logical sense.

But the first pilot episode of this BBC4 comedy screened back in 2010 got a mixed reception.

People who hadn't read Douglas Adams' original novels tended to like it more than devotees.

They were peeved that Dirk Gently wasn't played by a pudgy man wearing a red hat, a green striped tie and thick metal ­specs, but by Stephen Mangan.

Fans also objected to the way Howard Overman's script left out so much of the book's detail - which is a bit like complaining that you can't fit the entire British Olympic Squad on a push-bike.

Recommissioned for three episodes (they're nothing if not bold at BBC4!) Mangan returns along with Darren Boyd as his much put-upon partner Macduff.

It's a name that's perfectly suited to being chewed over and spat out with scorn as Gently does here.

Tonight Dirk must discover the connection between a man who thinks the Pentagon wants to kill him and another man who thinks his horoscopes are coming true.

According to Dirk's holistic view, these two seemingly unconnected cases must be linked.

And fans of Adams' novels will be pleased to see Dirk's theory of "Zen Navigation" comes straight from his book, The Long Dark Tea Time Of The Soul.

Basically, if you have no idea where you're headed, just find a car that looks like it knows where it's going and follow that.