By William Gallagher BBC News Online

The Shield has caused a storm in America because its lead character is easily the most corrupt and detestable police officer in television history.

That is great, but not yet great enough.

The previously unknown Michael Chiklis has become an Emmy award-winning star because of his role as Detective Vic Mackey and he deserves it.

Chiklis plays this brutal man brilliantly: he makes Mac feel like a real-life, all-powerful bully when he could have come across as a fantastic caricature.

Vibrant

The Shield puts Mac at the centre of the kind of messy story that feels vibrantly real and out of control.

You cannot compare The Shield to The Bill without explosive laughter

There is no pat ending in sight and the shifting interplay and nasty sense of danger is compelling.

The only problem is the risk that when you stand up one hugely effective character, he can outweigh everything else and you can end up looking to find controversy instead of telling a story.

The opening episodes of the show contain the best shock since Hill and Renko were shot in the closing moments of Hill Street Blues's pilot episode.

Mac is tremendous and his officer is exceptional too: he is played by Reed Diamond, best known for a rather longer running role on Homicide: Life on the Street.

But the rest of the cast is more variable and, at least in these early episodes, the show lacks the completeness that marked Homicide, the sense that every actor was top class, that every role was strong.

The Shield shows lots of promise

Homicide is the show this will most often be compared to - you cannot compare The Shield to The Bill without explosive laughter - because both shows aspire to a pragmatic realism about police work and police officers.

The Shield pulls it off almost all of the time but sometimes it is close to losing that realism through looking for things for Mac to do.

Profoundly vicious

In the first episode of The Shield after the pilot, for instance, Mac throws down a playing card with the name of his police unit, Strike Team, at the scene of a ruthless arrest and it feels wrong, somehow.

It feels like drawing a moustache on a portrait: Mac's true character is profoundly vicious and callous and this dents him.

These are early days for the show and it succeeds far more than it falters.

Just at the moment it is not quite the wonderful series it is said to be - but in America it has already reached its second season and there is every sign it gets better and better.

The Shield is on Five on Sunday at 2200.