A startling evocation of the beauties of nature, Sean Penn's film is a lyrical, tragicomic reconstruction of the journey of a young man who spurned society and his bickering parents to set off on a trek into the wilderness - or at least parts of America away from the main routes - heading for Alaska and his dream of a lonely kind of freedom.

It's persuasively put together, technically adroit and with a burningly intense central performance by the little-known Emile Hirsch. Getting away from it all is a common urbanite dream. Most of us are content to achieve it for a few weeks' break. For Christopher McCandless, a well-to-do, well-educated young man, it was a life vocation. The film is concerned with showing you how that might work out, balancing the bits that would appeal to most of us (freedom and the great outdoors) with those we wouldn't touch with a bargepole: there's a graphic sequence in which Hirsch kills a moose, amid vast amounts of blood, only to lose most of it speedily to maggots and decay.

McCandless kept a diary, but didn't film anything, so this is radically different to Werner Herzog's Grizzly Man, brought forcefully to mind when an enormous bear wanders past our hobo hero. Living the Jack London life - McCandless' great hero - is very hard work and in this case it includes a stint working in a fast-food joint and a serious kicking from a rail employee after Hirsch gets a hobo-style free ride.

The film is written by Penn, from Jon Kracauer's book, which retraced the central figure's journey. He seems pretty lucky in the company he finds - Catherine Keener, Vince Vaughn and Hal Holbrook are among their cinematic representations - and, for a long (142 minutes) film, this is a surprisingly engrossing one, inviting you to inspect your philosophy of life as well as that of the extreme, rather self-important version displayed by its youthful protagonist and making for a thoroughly unusual, non-judgmental and thoughtful entertainment.