Aliens always take such a condescending view of humans. On the planet K-PAX, Earth is described as a planet in “early evolution, future uncertain.” Smug talk from a planet whose name sounds like a radio station!

Those strange visitors from other planets are always eager to help, though (except when they’re eager to kill us). In the movie “K-PAX,” Kevin Spacey plays a man named Prot who may or may not be an extra-terrestrial, but who is full of sage advice for the Earthlings.

Presumed to be more crackpot than alien, Prot is put into a New York mental hospital, where his doctor is the kindly Mark Powell (Jeff Bridges). Mark is sympathetic, but he’s convinced he can figure out Prot’s psychosis and help him understand he’s delusional about the whole alien thing.

And yet, Prot seems to know a lot about outer space, and he can see ultra-violet light, which humans shouldn’t be able to do. Can this be explained, or could he really be an alien?

Will Prot help the other patients overcome their own problems, a la “One Flew over the Cuckoo’s Nest”? Will he help Dr. Powell gain a better perspective on his own life? Will he make a lot of soft-voiced Spock-like observations that show us all the ways we as a society have gone astray?

Do you really need to ask?

Kevin Spacey, consummate actor that he is, avoids the preachiness that would have resulted if, say, Robin Williams had played the part. He takes the role seriously enough to give an outstanding performance — particularly in a riveting hypnosis scene — but not so seriously that it becomes heavy-handed.

Jeff Bridges, for his part, is also good, as is Alfre Woodard as another psychiatrist. Director Iain Softley maintains a light tone, mostly, and keeps us guessing as to Prot’s true biological nature.

The trouble is in Charles Leavitt’s script (based on the Gene Brewer novel), which has an ending that tries to have its cake and eat it, too. That is to say, it’s ambiguous and unsatisfying. Keeping us guessing is good for a while, but ultimately, some things need to be answered. Maybe those really smart aliens will get some profound, inspiring message out of it, but this Earthling sure didn’t.

B- (; PG-13, a few profanities (including one use of The Big One), some violence and violent themes.)