Extra-terrestrial Prot is not your typical alien - he's got an ironic sense of humour and a working knowledge of sci-fi cinema history.

"Don't worry," he tells New York psychiatrist Mark Powell (Jeff Bridges). "I'm not going to suddenly burst out of your chest."

As well as playfully taunting Powell with that shock scene from Alien, Prot (Kevin Spacey) is also proving a bit of an enigma for the seasoned shrink.

Picked up by the police at Grand Central Station, he has insisted from the off that he's not human despite displaying all the usual human characteristics.

Powell's met a lot of so-styled "aliens" before but they normally trip up following a persistent line of questioning about their background and "home planet".

But Prot is different. Each question and inquiry is parried lucidly and convincingly, leading Powell to label Prot the "most compelling delusionist" he has ever met.

Now, if you thought Robert Kilroy-Silk occupied that particularly rarefied position then think again.

A medical examination reveals that Prot has an unusual vulnerability to ultraviolet light but the real clincher comes when he's invited to reveal his knowledge of the universe.

In front of a panel of esteemed astronomers, he perfectly illustrates a solar system (his own) so many light years from earth that the watching experts are barely aware of it.

Powell speculates it could be the conclusion of an idiot savant but Prot's profile doesn't fit this convenient explaining away of a growing mystery.

As hel digs deeper it becomes increasingly clear that some terrible episode in Prot's history proves whether he is indeed from outer space or a particularly convincing fraud.

To fully succeed, this sort of other-worldy plotline has to have completely believable characters and a literate and intelligent script.

K-Pax is blessed with both. Spacey is thoroughly convincing as the visitor, at one extreme, supremely comfortable with his elevated intelligence while occasionally awkward with human social conventions.

Bridges excels as the initially sceptical psychiatrist whose professional interest in his patient turns into a real friendship as he is asked to question is own existence.

The denouement leaves as many questions as answers, which is as it should be in a movie which leaves the usual sci-fi malarkey lights years behind.

Tim Evans