The Mercy (★★★,12A, 102 mins) is a film about a man who is first trapped in a dream, and then in a lie, and one watches it with a kind of queasy, vicarious anxiety.

It is based on a true story, that of Donald Crowhurst (Colin Firth) – a keen amateur sailor, struggling businessman and loving father of four – who decided in 1968 that he would compete in the first Sunday Times Golden Globe race. The organisers promised a trophy to the first person to sail alone around the world without stopping, and a £5,000 prize for whoever did it in the fastest time. The other competitors were the more experienced sailors, but Crowhurst – also an inventor of maritime equipment – saw an opportunity to showcase his innovations. Seemingly, he actually believed that he could win.

The film opens on dry land, at a boat show, with a fateful gleam in Crowhurst’s eye. The screen is bursting with the Devil-may-care optimism of the late 1960s: smiling girls in bikinis puff on cigarettes, and alcohol flows plentifully. Back home, in a sprawling but dilapidated house, Crowhurst tells his wife, Clare (Rachel Weisz), of his plans to build a boat, a trimaran, for the race. His adoring children are swept up in his buoyant enthusiasm. Clare has some doubts – Weisz’s mouth tightens at every mention of the race – but this was an era when a good wife did not contradict her husband by highlighting his deficiencies, but loyally applauded his efforts.

Firth is wonderful in the role, his expression perpetually shifting from faintly maniacal confidence to burgeoning unease. Crowhurst hires a journalist-turned-PR man, Rodney Hallworth (David Thewlis), who has a headline-generator in place of a heart. He also finances the race by borrowing from a shrewd businessman, Stanley Best (Ken Stott), staking the family home upon completion. Crowhurst departs from Teignmouth, Devon, on the last permissible date, in an unfinished boat, seen off by a brass band and cheering locals. By now, he is already appalled: he has built an imperfect craft but a perfect trap, which pits his reputation and the family’s security against an impossible feat. Once at sea, the icy reality leaks in very quickly. Losing nerve and direction, he later begins to falsify reports of where he is.

James Marsh’s film conveys great poignancy as it ricochets between Crowhurst, psychologically disoriented in the vast ocean, and his innocent, increasingly impoverished little family in England. The script strikes a clunky note, though, in giving Weisz an eloquent speech at the end which decisively blames the massed press and public for the tragedy that emerges.

The real story was even more complex: the wider world was indecently willing to urge it on, but the sorrowful delusion began at home.

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