*****

In The Dark Knight Heath Ledger has left behind a professional legacy that not only demonstrates his superb talents as an actor, but the sad loss of his potential.

Much has been said in the lead-up to the release of the latest Batman film regarding the recognition of Ledger's performance in the form of a posthumous Academy Award nomination.

For the audience such discussions can be frustrating as it puts an extra level of pressure on their perception of Ledger's role as Batman's nemesis The Joker.

However, in The Dark Knight, Ledger's carefully considered performance is simply outstanding. With his creased, smudged make-up, rapid hand movements, nervous facial tic and no real indication as to how he reached his state-of-mind, Ledger's Joker has all the uncertainty of an out-of-control drug addict.

Yet, as he teeters on the brink, there is an underlying rationality to Ledger's characterisation that is real, compelling and thoroughly engaging. The Joker asks us what we could do if we lost our morality.

Morality is the key to The Dark Knight as Gotham City's ongoing struggle with criminals continues and those on the right side of the law have to question their own decisions and motivations.

Bruce Wayne as Batman (Christian Bale) remains the city's guardian and works closely with Lt James Gordon (Gary Oldman) to rid the streets of the criminals who are becoming smarter, more dangerous and have many of the city's allegedly upstanding citizens on their payroll.

The fact the new high-flying district attorney Harvey Dent (Aaron Eckhart) has been employed to work alongside Wayne's childhood sweetheart Rachel (Maggie Gyllenhaal) at City Hall is a beacon of positive light for all. Dent made his name jailing corrupt cops and he is perceived the hero - and certainly the face - of a new, safer Gotham.

Yet, the tide had shifted in the underworld as the criminal fraternities are being replaced by one man: The Joker. The Joker's strength does not come from his refusal to work within the traditional bounds of morality, but the fact he has no limits.

His spree is simply a means to get to Gotham's legend and to achieve this he pits, and successfully manipulates, the residents of the city against their caped crusader and demands that Batman be unmasked.

This action raises a central concern of The Dark Knight, which explores whether Batman is a crusader or a vigilante who has taken his powers too far? And, if the latter is true, has Batman forced the lawless into darker areas?

Yet, for the audience, there is another side to The Dark Knight and that is the viewing pleasure you get from the two-and-a-half-hour cinematic experience.

For action buffs, there's no respite as one action sequence leaves you with little time to recover before the next step in plot development.

The Dark Knight is a fun, funny, exciting and, with its darker edge, very much in the vein of Batman Begins.

The whole cast gives terrific performances but there is no doubt about it, this is Heath Ledger's film. Sadly, there can be no encore.