I believe in Harvey Dent. So proclaims the election campaign for Gotham City's new District Attorney. After years of living under the tyranny of mobsters and corrupt officials, a true hero has emerged – a man of the people with a plan to take down the criminals, to enforce justice, and most importantly, a man with a face. The people of Gotham no longer need to put their fate in the hands of a masked vigilante who hides in the same shadows as those who threaten them. Batman (Christian Bale) is presented here as a man driven by his own twisted sense of justice and vengeance. He sees his childhood sweetheart Rachel Dawes (Maggie Gyllenhaal) fall for Dent (Aaron Eckhart), a man who represents everything Batman wanted to become, and finds himself pitted against The Joker (Heath Ledger), a diabolical villain with no agenda beyond wreaking havoc, inciting anarchy and burning Gotham City to the ground.

Director Christopher Nolan's second outing for the Caped Crusader is an unrelentingly downbeat vision. He presents Gotham City as a gritty, urban hell populated by demons who terrorise the streets and plague the consciousness with equally destructive affect. His characters are larger than life, torn as they are from the pages of comic books, yet there is nothing "comic" about the world that they inhabit. That is not to say the film is gruelling, The Dark Knight is as thrilling as even the most rabid fan boy could have wished for. Yet, there is a sense of tragedy and failure that weighs heavy on the shoulders of these characters.

From the opening Bank heist, where The Joker is introduced not only as a mischievous malevolent sociopath, but also as an adept and highly dangerous criminal, the film unfolds at a breakneck pace. The action follows Lieutenant Gordon (Gary Oldman), Harvey Dent and Batman as they struggle to take down Salvatore Maroni (Eric Roberts) and his network of gangsters, even when this requires Batman to leave Gotham for Hong Kong in order to retrieve a dirty bookkeeper. This flying visit East might eventually prove inconsequential to the plot, but it does give cause for some beautiful vistas of Central at night, (So well done those who left their lights blazing as requested) and helps showcase Nolan's dazzling use of IMAX photography. Meanwhile, the Joker instils fear in the public and the mobsters alike as his unique brand of chaos runs riot in the city. "This city needs a better class of criminal," he declares. In doing so, he comes face to face with Batman, only to feel a karmic bond grow between them.

The script is filled with menacing characterisation and quotable dialogue, as well as half a dozen or so beautifully staged action sequences, including a fantastic city centre hijacking that introduces audiences to the agile yet battle-ready Batpod. In the third act, the film offers up all manner of twists to surprise its audience – even those who have followed the film's ingenious viral marketing campaign. But while some elements of the plot might become as implausible as they are presumptuous, it is the acting more than anything that keeps the audience engaged right until the decidedly bitter end.

Praise for Heath Ledger seems almost clichéd now, but his final completed film performance, as The Joker, is a genuine tour de force. Ledger is unrecognisable, immersing himself completely in a role that will instantly become one of the great screen villains. There is already awards buzz surrounding his performance as the psychopathic clown, which can only further highlight that the greatest tragedy is Ledger's not being here to relish the best reviews of his career. The rest of the cast – which includes Christian Bale, Gary Oldman, Aaron Eckhart and Michael Caine – are all perfectly fine, but are ultimately pushed into the shadows by Ledger's menace to society.

What Nolan has delivered is a summer blockbuster of deep moral complexity and one of the darkest superhero incarnations audiences have ever seen. When held up against the likes of Iron Man or The Incredible Hulk, the competition seems childish and lightweight by comparison. The Dark Knight asks its audience to side with a hero of dubious origin, with questionable methods and unclear aspirations. A hero perhaps no better than those he strives to vanquish. For as Bruce Wayne himself points out, "Batman has no limits – and that's his strength."

James Marsh