Kevin Spacey plays Prot who is from K-Pax (or IS he?) and who gets picked up by the NY police and then sent to the looney bin.
Unlike most of this sort of movie where we are taught the truths that only the gentle inmates can see, this actually focuses mostly on whether Prot is really an alien and the psychiatrist's (Jeff Bridges) efforts to solve the mystery of what trauma caused him to form such a perfect false persona to hide behind. Also, we see Jeff Bridges' alienation from his family due to work and generally being a jerk to his wife (no wonder she is his second one) ... this part is fairly shallow and predictable.
For me the most interesting thing was whether Prot was an alien or not. Well acted, well shot, and well edited.