Magnolia matters, because Magnolia duplicates that pitch for most of its bladder-threatening 188 minutes; it’s a film about the quiet desperation that surveys most lives. ‘It’s not going to stop’, sings Amiee Mann on the soundtrack, ‘so just give up’. Things don’t always get better, this sprawling, disjointed, bitter film seems to say. The best you can hope for is that they change, that the unlikely is still possible. If Tony Marchant’s astounding BBC series Holding On hadn’t got there first Anderson could have laid claim to its title: the two share a nerve-shredding fascination for putting the audience through the emotional ringer.

‘I’ve set out to make the mother of all San Fernando Valley films,’ Anderson’s quoted as saying. ‘I may succeed because the only competition seems to be myself.’ Magnolia (named for the street that runs east-west through the Valley) smacks of the picture he had to make, his grand folly. The obvious comparison is to Altman in general, and Short Cuts in particular. That film and this share Los Angeles, large ensemble casts and virtually the same screen space. Let's go out on a limb, though, and suggest that Anderson's is the better of the two.

He is, after all, only just thirty and this is only his third feature, and yet already his writing demonstrates an astonishing maturity. It was as true of his debut, Hard Eight, as Nights - Anderson is not a film-maker prepared to rest on irony. And for such a young man, Magnolia is a film quite unashamed to foreground failure and mortality.

Dying TV producer Earl Partridge (Jason Robards) asks his nurse (Philip Seymour Hoffman) to find his estranged son, Frank. Frank TJ Mackey (Tom Cruise), motivational speaker (‘Tame the cunt!’) behind the misogynist Seduce and Destroy. Partridge’s distraught trophy wife (Moore) married him for his money, but now she realises loves him.

One of Partridge’s shows - What Do Kids Know? - is presented by Jimmy Gator (Philip Baker Hall), himself both dying from cancer and estranged from his own drug addicted daughter (Melora Walters). She meets policeman Jim Kurring (John C Reilly) when he’s called to her apartment. He asks her out. Elsewhere, Donnie Smith (William H Macy) laments his early life as a successful quiz kid. Now all he wants is the love of a local barman. ‘My name’s Donnie Smith and I have lots of love to give.’

Visually Anderson is maturing at an exponential rate. While he could still be tried for crimes against Stedicam, the debt owed to Martin Scorsese has all but given way to something equally charged but altogether more personal. Magnolia is an epic of the intimate, fuelled (and think on this) by arguably the best ensemble cast ever assembled: there are outstanding recent talents (Hoffman, Moore, Macy, Reilly); old stagers at the top of their game (Hall, Robards); and jaw-dropping turns from Cruise and the all but unknown Walters. Even Altman had to contend with the risible Andie McDowell.

Musically too, Anderson takes ever more risks by having much of the picture sung-through by the low-key, conversational Mann. The effect, alongside Supertramp and the deliciously ironic (sic) use of Gabrielle's 'Dreams', is of a Greek chorus. His only serious mistake is here also - mixing Jon Brion's otherwise fine score too high for the first hour. The effect, while it does admittedly focus the attention, is of a radio left on in the background. He does himself a disservice.

But what a small price to pay for a film of rare perception and depth. Seldom do quality and quantity make for such good bedfellows. Anderson is a consummate film-maker, prepared to trust his instincts, even if those instincts push him to nakedly breech cinema’s fourth wall (rain stops on cue, his cast sing along to the radio in a heart-stopping montage). Magnolia is exactly the film we wanted PT Anderson to make after Boogie Nights - precocious, compassionate, ambitious, magnificent.