We first meet Firth’s Crowhurst as he’s desperately trying to sell his nautical navigation device the Navicator. Punters won’t bite, and there’s a sense that his business is failing. Crowhurst himself is an amateur weekend sailor and, inspired by Francis Chichester’s (Simon McBurney) around the world journey, he decides to enter the 1968 Sunday Times Golden Globe Race in a bid to claim the prize money and turn his fortunes around. What’s great about The Mercy is how it manages to surprise, both in the story it’s telling and how Marsh gradually shifts the tone. Crowhurst is initially painted as a dreamer, a plucky underdog who’ll conquer insurmountable odds to circumnavigate the globe. “If I can do it so can the bloke who stares at the horizon,” he says at one point. It’s all a bit twee and snoozy, but Marsh and screenwriter Scott Z. Burns are cleverly sewing seeds of doubt. Crowhurst suspects he’s unprepared for what’s ahead, and once he’s out at sea the film pushes into darker more psychologically complex territory. Stripping away the bluster and self-aggrandising, Crowhurst is alone, isolated and battling with his leaky vessel. It should be said that Firth gives an absolutely brilliant performance here. On land his Crowhurst is a mix of charm, idealism and delusion; at sea he’s frightened, alone and mentally unraveling. Despite his nautical deception, there’s never a moment when you lose empathy for him. The Mercy reminded me a lot of J.C. Chandor’s All Is Lost and Marsh’s earlier film, the extraordinary documentary Man On Wire. The former for its situation, a man against Mother Nature scrambling to patch up his battered boat, and the latter for resurfacing a fascinating piece of history about a man with grand ambition. Crowhurst and Twin Towers high-wire artist Philippe Petit are two very different people, but in both cases Marsh is eager to get behind the showmanship and ask a simple question: why did they do it? The Mercy is in UK cinemas from Friday.