2002′s Infernal Affairs was one of the best and smartest cop thrillers of recent years, with a premise so irresistible that it’s surprising nobody thought of it earlier. A special police department are working to bring down the local crime-lord, and have managed to infiltrate a mole into his organization; but the gangster, not to be outdone, has his own mole in the department that’s tracking him. As the plot develops, both moles race to discover the identity of their counterpart before their own cover gets blown.

The news that Affairs would be getting an American remake was hardly surprising. But that it should be Martin Scorsese, arguably the greatest living American director, at the helm … well, that was interesting. Surely a director of Marty’s caliber wasn’t leaping onto the bandwagon of pillaging the Asian film industry?

Scorsese has gone on record as saying that The Departed is not a remake, per se, which presumably would explain the change of title and place it in that most dubious of modern movie categories, the “re-imagining.” But thankfully, Mr. Scorsese was lying through his teeth. The truth is that The Departed is unarguably a remake – in places shot-for-shot, word-for-word, scene-for-scene. William Monahan’s script changes a little, expands a lot, but in no way has it forced its source material through a meat-grinder as Ring did. Beneath the Scorsese sheen, beneath the astonishing amount of swearing, the heart of Infernal Affairs remains firmly intact.

And this is both a good thing and a bad thing. But good points first: Having his premise already in place has allowed Monahan to focus on polishing up rough edges, and his script is routinely superb; witty, nasty, rhythmic and emotionally complex. Most of the changes come at the start, where Monahan has inserted about an hour’s worth of character development, providing plenty of material for an astonishing cast to get to grips with.

Jack Nicholson’s performance as gang boss Frank Costello goes as wildly over the top as you’d expect, but is no less brilliant for it; and for all its excess, there’s a subtlety and humanity to it that has been missing from Nicholson’s work for a long time. But DiCaprio, as the police mole in Costello’s gang, though less showy is arguably better. His Billy Costigan is a fundamentally good man knowingly doing evil and driven to the brink of madness by it, and DiCaprio conveys bottled-up rage and pain better than any other actor of his generation could hope to.

With two performances this good, it’s impressive that the rest of the cast hold their own, and another credit for Monahan that he allows even fairly minor characters plenty of space to breathe. Alec Baldwin, Martin Sheen, Matt Damon, and particularly relative newcomer Vera Farmiga as the psychiatrist in love with both moles, all manage to bring depth and complexity to their parts. And Mark Wahlberg deserves a special nod for an astoundingly funny and foul-mouthed turn – his abuse of the English language is revelatory, and even his hairstyle is funnier than most comedies this year.

But what about Scorsese? Some people are calling this a return to form, but it’s perhaps fairer to call it a return to home turf; he’s in his comfort zone and his direction is assuredly brilliant. Arguably, though, he’s at his best in the first half, which allows him to make the material his own; later scenes are remarkably faithful to the source material, which is commendably respectful but does beg the question of how much credit Scorsese can ultimately be allowed.

It’s perhaps an unfair criticism, since most people won’t have seen Infernal Affairs and be judging by comparison. But then, if you have seen both films, it’s a comparison that’s impossible not to make. And where Affairs worked as a slim, fast-moving procedural, The Departed feels bloated in comparison. The plot literally doesn’t start until half way through, and yet in many ways it feels like a moral simplification; where the Hong Kong film traded solely in shades of grey, with no truly bad or good guys, the remake makes Damon’s mole a lying scumbag and Nicholson’s gang boss essentially a monster.

It’s fair to say that, standing on its own, The Departed is certainly one of the year’s best movies; and even viewed as a remake, it’s still superb, in many ways superior to the original. But as a thriller, it lacks brevity, and as a Scorsese film it lacks complexity and – most of all – originality; it retreads ground that we would normally expect to see him breaking.