THIS earnest biopic from director James Marsh (The Theory of Everything, Man on Wire) takes a surprisingly sombre look at one man’s amazing true story which belies the potential Oscar-bait mould that its outline and filmmaking pedigree might suggest.

It tells the real-life story of Donald Crowhurst (Colin Firth), a middle-class but financially struggling family man filled with optimism as he attempts in vain to sell self-made boating equipment at an exhibition with his family by his side. It’s there he hears of a yachting competition that challenges amateur and professional sailors to a new solo, non-stop race around the world.

Much to the surprise of his supportive wife Clare (played by the always-brilliant Rachel Weisz, pictured centre), mostly because he has hitherto only really been a weekend sailor, he signs himself up for the challenge. “Alone on a boat for nine months, you’re either drunk or mad,” disbelieving friends tell Crowhurst. “Well, we should have another drink so we can rule out madness,” he casually replies.

After strict preparations, including the construction of a new fully equipped sailboat nicknamed the Teignmouth Electron and adorned with the logos of sponsors conjured up by savvy press agent Rodney Hallworth (David Thewlis), he embarks on the race full of hope and promise.

However, part way through the expedition he discovers that he’s not quite as prepared (either physically or mentally) as he first thought. After hitting one disaster after another, he realises he can’t complete the race. But instead of admitting it to the world and returning home, he concocts a deception to convince everyone via faked radio reports that he’s doing a lot better than he actually is. Needless to say his dire situation and the lie spirals out of all control.

Firth is predictably engaging as Crowhurst, playing him with his usual likeable British charm that has made him such a draw for audiences and which feels perfectly cast for the real figure’s initially carefree, almost naively confident persona. But it’s far from coasting as Firth’s performance brilliantly morphs from that can-do exuberance to a believable decline in mental stability without it ever coming across as trite or showy.

It’s heart-breaking indeed to see him slowly lose his mind as his predicament worsens as the weeks go on; he can’t go back lest he lose his family home that he put up as collateral for the sponsors, he can’t embellish the truth too much because it would arouse too-good-to-be-true suspicions from the folks back home.

He’s a small fish caught in the extremes of a very big pond and the net of inner turmoil – enough to make anyone go round the bend. Respect the sea and respect man’s limitations even more seems to be the film’s mantra, whispered into its atmosphere of melancholic tragedy.

The drama is less successful when it focuses on what’s going on back home, coming across as clichéd filler to pad out the expected angles of a standard biopic. Weisz’s great performance brings depth to the otherwise simplified role of Clare, while the commentary on the nature of newspaper sensationalism and the need to build up and tear down people in the public eye doesn’t quite land as it should.

But there’s a sincerity to Marsh’s biographical retelling that’s commendable, and a thematic ambiguity that’s intriguing. You’re left to decide for yourself whether Crowhurst was a hero bravely venturing out to achieve the seemingly insurmountable, or a gullible fool who left his family behind to chase a pipe dream, often enigmatically blurring the line between the two.