Back in the summer, the cinema release of The Dark Knight sent fans and critics into a frenzy: box-office records tumbled and superlatives gushed. Yet four months on, now that the delirium has abated, it’s clear that Christopher Nolan’s grim follow-up to Batman Begins has been wildly overrated, notwithstanding its position at No. 4 in the IMDb’s all-time greatest movies poll. As blockbuster action films go, The Dark Knight is good, but it’s not that good.Heath Ledger’s mesmerising, scene-stealing performance as the Joker, on the other hand, is as great as everyone said. For all the gazillions lavished on the movie’s special effects, when The Joker is on screen, you can’t look at anything else. Forget Jack Nicholson’s cackling, campy Joker in Tim Burton’s 1989 Batman; Ledger’s anarchic psychopath is the real deal.With his smeared whiteface make-up and leering razor-slashed smile, Ledger’s Joker is creepy enough to look at, but what he represents is even more chilling. “I’m an agent of chaos,” he declares as he unleashes a wave of terror on the streets of Gotham, robbing Mafia-controlled banks, assassinating figures of authority and threatening mass murder. Ruthless and implacable, he is as big a threat to the underworld as he is to the forces of law and order represented by Aaron Eckhart’s crusading District Attorney Harvey Dent and Gary Oldman’s upright cop Jim Gordon.To stop him, Christian Bale’s Batman has to resort to ever more extreme measures, including a spot of extraordinary rendition (kidnapping Mob banker Lau from Hong Kong) that inescapably calls to mind the West’s War on Terror.From here it’s a small step to the comparison made by some of The Joker with Osama bin Laden, another remorseless foe with whom it is impossible to negotiate. The analogy soon breaks down, though. Bin Laden may instigate unpredictable terror, but he does have a purpose; the Joker derides everyone with a plan. He desires chaos not a universal caliphate.The Joker’s anarchy is terrify