Philip Seymour Hoffman seems to be everywhere these days. Whether it’s as a weaselly hack in Red Dragon (2002), a phone-porn magnate in Punch Drunk Love (2002), or a sexually repressed school teacher in 25th Hour (2002), PSH has become known as one of the most talented supporting actors in the business. He’s even found time to direct a play for the London stage. And now, finally, he gets his first fully-fledged leading role in this excellent new film about gambling addiction, based on a true story.

It begins with an exchange between Hoffman’s character, Dan Mahowny, a dull but successful Toronto bank manager and his psychologist. "Everyone has a public life, a private life and a secret life," says the psychologist. "Thing is, my secret life is a little less secret than everyone else’s right now," replies Mahowny. The film goes on to show why.

There is no complex plot to follow here. The film simply follows Mahowny’s spiralling addiction, as his gambling moves from small-time bets on horses or basketball results to ever larger sums spent in Atlantic City and Las Vegas casinos, financed by millions of dollars filched from clients’ bank accounts. He becomes a top of the range high-roller without living the lifestyle. He drives a clapped-out motor, he drinks cola, eats ribs at the casinos and dresses abysmally. But his focus is only ever on the gambling, with desperate calls to dodgy bet-laying associates and ever more crooked ways of finding more money. Each trip to the casinos creates a bigger debt to pay off that can only be solved by massive fraud at the bank.

He becomes increasingly popular at the Atlantic City casino, who offer him ever grander perks, which he doesn’t really care about, so long as he continues to play at their tables. Some of the lesser employees even seem to start rooting for him to win at the expense of their employer. John Hurt gives a superb performance as the oily, amoral casino boss. His eyes seem almost to have cartoon dollar signs rolling around in them every time Mahowny takes to the tables. It’s a joy to watch him flinch when he thinks Mahowny might clear them out.

But of course he doesn’t. He has umpteen opportunities to pack it in, or to quit while he’s ahead. At one point he $9 million up, but by the end of the night he’s lost it all – just as Hurt’s character predicts. The impact on his life is initially minimised as he keeps his addiction secret – or rather just doesn’t acknowledge that he has a problem. The only person who realises the problem is his bank clerk girlfriend (played by Minnie Driver in a badly-made blond wig), who becomes increasingly frustrated by his unreliability and lack of attention. He even contrives to take her off on a "romantic" weekend trip to Vegas. She naively thinks it’s to get married, because "why else do people go to Vegas?"

Seymour Hoffman is magnificent in portraying Mahowny’s triple life: quietly efficient at the bank, awkward and affectionate with his girlfriend and then scared but totally driven as a gambler. Most of all, we sense the all-consuming nature of his addiction. He’s preoccupied with what he calls his "financial problem" all the time. He manages to outwit the auditors at the bank but is finally caught by a detective who’s actually pursuing his dodgy bookmaker associate. The story passes into legend – hence this film.

The pacing of the story is perfect as the excitement levels rise in parallel with Mahowny’s rising debt burden and criminality. The film is skilfully edited by Mike Munn and never loses momentum or interest; but above all this film has some great acting, Minnie Driver aside. Seymour Hoffman finally gets the lead role he’s worthy of, and John Hurt supplies an excellent counterpoint as the Mephistopheles figure.

The film closes once more with an exchange with the psychologist. Mahowny rates the high from gambling on a scale of 1 to 100 as 100. Asked by his shrink what score the next-best high would be, Mahowny replies "20." Can the real Mahowny get through life living at just 20? As the film implies, only time will tell.