Directed by Garth Jennings, starring Martin Freeman, Mos Def, Sam Rockwell, Zooey Deschanel and John Malkovich

This long-awaited film version of the venerable radio series/book/tv series comes with the weight of considerable expectation from generations of devotees.

The plot (such as it is) revolves around a grumpy Englishman named Arthur Dent (Freeman of 'The Office' fame), who is dismayed to discover that his house is to be demolished to make way for a bypass.

His dismay is quickly trumped by the discovery that the entire Planet Earth is to be vaporised to facilitate an interstellar bypass. The only hope of survival for Arthur is his friend Ford Prefect (Mos Def), who turns out not to be from Guildford as Arthur had always thought, but is in fact an alien and roving researcher for an electronic travel book, called 'The Hitchhiker's Guide to the Galaxy'.

Having hitched a lift on board the Vogon ships that destroyed Earth, Ford and Arthur embark on a voyage across the galaxy in the company of Galactic President Zaphod Beeblebrox (a lame Rockwell) and his female sidekick Trillian (Deschanel).

Adapting such a well-loved book is always tricky, and when the film's plot diverges from the book's original narrative, it seems to wobble a bit. But there's little point in grumbling about Hollywood hacking up your favourite masterpieces – the changes were made by the book's author, Douglas Adams, before he died. Not that this has stopped a small clique of online fanboys jumping on the slightest deviation from the original 'sacred text', a fate it would seem that is suffered by any adaptation of a well-known work these days.

Lack of faithfulness to the original is not this film's problem. Instead, the tricky business of finding the right tone has proved extremely difficult.

The fast-moving, eye-popping sci-fi universe (decently rendered by the effects team), and the Monty Python-style Oxbridge wordplay of the humour make for an uneasy marriage. Some of the gags are still funny, but many are painfully dated (what's a Ford Prefect?) and laboured, and it's difficult for them to find space to work among all the whizz-bang effects and multiple plot twists.

Martin Freeman is great in the Dent role, making an excellent fist of the tricky business of being cantankerous and clueless, but still likeable. Mos Def is passable as Ford Prefect, and though his take on the character is miles away from previous incarnations, it somehow works.

Sam Rockwell has the thankless task of portraying Zaphod Beeblebrox, but he suffers from a tragic charisma deficit. His irritated, semi-disengaged performance brings nothing to the role. John Malkovich is simply wasted.

Zooey Deschanel is the pick of the actors, luminously appealing in the expanded role of Trillian. The script beefs up the Zaphod-Trillian-Arthur love triangle by placing it centre stage, adding some necessary dramatic weight to the film's plot and letting Deschanel's charm shine.

Visually, music video director Garth Jennings brings a good sensibility and creative energy to the project. There are plenty of memorable scenes and characters (the planet-building zone in the interior of Magrathea, Jim Henson's Vogons) but the overall package is unsatisfying and barely coherent. The blatant sequel-trailing of the ending doesn't help.

It's not really funny enough, or dramatic enough, but given that 'The Hitchhiker's Guide to the Galaxy' might just be an unfilmable book, this entertaining, if patchy, attempt deserves a not-exactly-stellar thumbs up.

Luke McManus