If a man claiming to have travelled 1000 light years on a light beam from the planet K-Pax landed on earth tomorrow, the chances are we'd throw him into the nearest loony bin. There he would remain until he could show us his blood was green or he performed some alien tricks.

Such is the premise of this New York set, CGI-free fable. When psychiatrist Dr. Mark Powell (Jeff Bridges in good form) meets his new patient, he naturally assumes he's dealing with a "delusional". There is little evidence to suggest otherwise, although the fact that his patient has not responded to medication is unusual.

Sure Kevin Spacey's sweet-natured protagonist, Prot, has his eccentricities. His arms don't move much when he walks, he eats a banana with the skin on and wears a pair of dark sunglasses all the time (something to do with the fact that K-Pax isn't as bright as Earth) - but he looks human (well, he looks like Kevin Spacey doing his dead pan bit). Prot, as he calls himself, also proves to be intelligent, articulate and amusing: "Don't worry, I'm not going to leap out of your chest," he quips at his first therapy session.

Prot's story would, of course, be written off as pure madness were there not some science to support it. In this case, the filmmakers spice the plot with just enough information to keep the door of possibility open. Lab reports show that Prot can see ultraviolet light, the spectrum of light that is invisible to the normal human eye. Further along in the film, Prot reveals an uncanny knowledge of the planetary orbits in the constellation Lyra, home of K-Pax, to a group of stunned astro-physicists.

As the therapy contines, the more credible Prot's story becomes. Pretty soon he has all the other patients in the infirmary (think One Flew Over The Cuckoo's Nest) believing that he will take them back to K-Pax. While Prot goes about life with what seems a serene inner peace, Powell obsesses on this one case to the detriment of his family life.

Gene Brewer's script dips and weaves around the burning question - who or what is this guy? - leaving behind a cloud of wit, wisdom and possibilities swimming like motes in the light. If you are searching for the burden of proof, you will be disappointed, possibly irritated. This is not the X-Files, but something in a more humanist vein, where an investigative question like - "Could you draw a picture of a K-Paxian?" - is replaced by a more soul-searching question like "What are your obligations to other beings?"

The tension between whether he is or isn't also gives rise to some laugh-out-loud comedy like when Prot speaks his dialect, a language so complicated that it makes Klingon sound like Esperanto, or carries on a conversation with a dog.

On an emotional level the film covers a good deal of ground, with changes in mood from light to dark reflected effectively with heavily tinted cinematrography.

Much credit must also go to Kevin Spacey. Although he plays a largely serious role, he also proves to be a natural at deadpan comedy. He carries off the ambiguousness of Prot's character with understated precision.