With his latest film, director Iain Softley begs us to ask whether “prot” — the defiantly lower-cased name of Kevin Spacey’s mysterious mental patient — is from another planet. What we’re more likely to wonder is whether Spacey himself is from another planet.

It is difficult to believe that the same Kevin Spacey who gave us Roger “Verbal” Kent (“The Usual Suspects”) and Lester Burnham (“American Beauty”) is now prot, a moralizing, mystical alien from the planet that shares its capitalized name with the film, K-PAX. Certainly some alien influence came over him and his excellent supporting cast to persuade them to take part in this pseudo-intellectual morality tale that has the emotional punch of an episode of “Full House.”

True to earthly superstition, prot arrives, in a ray of light at Grand Central Terminal, where he is promptly arrested, presumably for being weird, and deposited in a mental institution. Mysteriously enough, there is no organic evidence of his dementia; prot convincingly tells his tale of faraway solar systems to Dr. Mark Powell (Jeff Bridges), who heads the hospital. As prot unveils more of his story, Powell and his skeptic human friends waver between doubt and belief, while prot’s fellow mental patients happily turn him into the most popular extraterrestrial in the ward.

Here Spacey lays the quirkiness on thick. His alien is charming and quite human, but gently betrays his unusual origins. prot has an unadulterated love of earthly produce, which he devours, peels, stems, leaves and all, in quick little bites. But outside this nervous tic, prot wears a peaceful, beaming smile that of course comes with transcending human trivialities, much like the spiritually enlightened souls on “Touched By An Angel.”

A slow and steady revelation of prot’s enlightened state occupies over half of the film’s running time. At first it’s almost clever, such as prot’s explanation of his ability to travel through beams of light, or the painful process of K-PAXian reproduction. The formulaic plot of a unique visitor who observes and enlightens is familiar. “K-PAX” is largely a hybrid of “One Flew Over the Cuckoo’s Nest” and “12 Monkeys,” but it does manage to add some originality to the scheme.

But the more times prot refers to Powell and our species as “you humans,” the more moralizing “K-PAX” becomes. It all starts when prot matter-of-factly says, “It’s hard to imagine you’ve made it this far,” and classifies Earth as a BA-3 planet — that is, early stage of evolution, future uncertain.

K-PAX, according to prot, is better than Earth because it has no need for laws or families. Unsurprisingly, prot slowly starts to embrace human values in a series of supposedly emotional moments, clearly copped from reruns of “7th Heaven.” The spaceman makes a connection with Powell’s daughter when he pushes her on a swing. He shows human tenderness when he advises Powell to appreciate the world around him instead of asking so many pesky questions about K-PAX.

Softley wants to use these contrivances to get us to question our values and then quickly and wholeheartedly readopt them in one glorious celebration of human sentiment. But Softley cannot dig to the psychological depths necessary to question those values, nor can he reach the emotional pitch required to reaccept them.

“K-PAX” is a far cry from Softley’s charged adaptation of Henry James’ “Wings of the Dove.” While that film had a distinct message about the unforgiving shackles of social convention, “K-PAX” doesn’t say much of anything. Much like prot, it only offers canned moral advice and a glossy grin.

Since the film fails to reach a deeper level, the actors are helpless. Spacey does an excellent job, briefly showing the emotional prowess he displayed in “Swimming With Sharks.” But usually he can do nothing more than what the script requires — that is, to be the mystery man with the beaming smile, your now typical benevolent alien. Though he plays the role well, the movie doesn’t measure up.

Jeff Bridges is equally natural as Dr. Powell, essentially playing Scully to Spacey’s Mulder. Powell is the perennial skeptic — he’s seen veritable proof of prot’s origins, but he still refuses to believe. Instead he feels compelled to find a moment in prot’s past when everything went wrong, to find a parent or a disaster, and to quell everyone’s doubts. This plot thread is thin and resolved quickly and ambiguously before it ever picked up steam.

The conclusion of “K-PAX” is much like the rest of the film: uncertain of what it wants to be. It is only sure of its contrived celebration of family values. Prot’s prescience is wasted — like the psychological and deeper emotional threads of “K-PAX” — on a film that wants not to challenge, but only to accept.