Who knew Sean Penn had it in him? The famously pugnacious paparazzi puncher and former Hollywood bad boy has here, with his fourth directorial feature, produced a sublime meditation on human happiness that’s user-friendly, visually ravishing and packs a hefty emotional wallop.

Adapting Jon Krakauer’s back-to-nature bestseller Into the Wild, Penn tells the beguiling real-life story of a Middle American rich kid called Christopher McCandless (played by Emile Hirsch) who, in the summer of 1990, exchanged a white collar future in Harvard Law for a penniless two-year trek across the American Wilderness. McCandless donated his college fund to charity, burnt his remaining cash, and set off on a raw-knuckle odyssey that would ultimately culminate in a brutal epiphany, alone and starving in an abandoned bus in Alaska.

The journey, however, is the thing, and Penn displays a remarkable lightness of touch as he depicts, in a non-chronological whirl, McCandless’s free-roaming adventures. Thus intercutting the celebratory freedoms of river rafting and Pacific Coast hiking with the tough Alaskan days of hardship and even tougher flashbacks to life with Chris’s stiff and strained parents (essayed with brittle finesse by William Hurt and Marcia Gay Harden), Penn builds up a mesmerising portrait of a young man who is part philosophical seeker and part embittered child.

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Along the way the characters that McCandless encounters, including Catherine Keener’s melancholic hippie and Vince Vaughn’s wildcard farm hand, are mostly benign and nurturing, and seemingly reflective of an entirely altruistic national psyche (possibly an apologia from a director often accused of being a seditious pinko traitor by the American Right). They also provide McCandless with a sounding board for his philosophical enquiry, which is here an intriguing salmagundi of Henry David Thoreau, Tolstoy and Byron – the film, in fact, opens with a scene-setting quote from Byron’s Child Harold’s Pilgrimage: “By the deep sea and the music in its roar; I love not man less, but Nature more.

During all this, and despite an electrifying central turn from Hirsch, Penn the director remains the film’s real star. For nothing in his previously gritty and slightly macho output (see The Crossing Guard and The Pledge) suggested a director who could push, as he does here, breath-taking nature cinematography to its limits – one shot, in the Sea of Cortez, a snappy pan from McCandless’s kayak down into the water to two dolphins swimming underneath - is typical of Penn’s bravura choices. Elsewhere he uses potentially distracting devices like split-screen storytelling and old 8mm footage with seemingly effortless élan.

Ultimately, though, Penn’s boldest move as a director is his straight-faced depiction and exploration of McCandless’s spiritual quest, which is essentially to discover the secret of the good, virtuous and happy life. It is, of course, deeply unfashionable to ask this kind of guileless philosophical question in a battered world imbued with 21st-century cynicism, but if there’s one grisly director with enough conviction to do it then it has to be Penn. The answer that emerges in the film’s closing and deeply moving minutes is simple and familiar, but no less profound because of it.

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