A mysterious person seemingly appears out of nowhere at New York's Central Station, stepping in to help a woman being mugged. His odd responses to police questions lead to his arrest. Claiming to be an extraterrestrial named Prot from the planet K-PAX, located 1,000 light years away in the Lyra constellation, he is subsequently committed to the Psychiatric Institute of Manhattan. Here, psychiatrist Dr. Mark Powell takes on the task of understanding Prot's unusual claims. Throughout their sessions, Prot impressively exhibits an uncanny ability to articulate his experiences regarding K-PAX and its civilizations. Notably, he can see ultraviolet light and is unaffected by medication, which only strengthens his narrative.
Prot asserts he is 337 years old and arrived on Earth 4 years and 9 months earlier, aging seven years during his journey at six times the speed of light. He describes K-PAX as a family-free realm devoid of unpleasant reproductive practices, laws, and any form of punishment for wrongdoing. Meanwhile, Mark appears to be struggling with his family life; he is married to Rachel Powell but is facing neglect issues at home. Mark's prior marriage ended in divorce and he has a son named Michael Powell who currently attends college but has distanced himself from his father.
As Prot's enigmatic presence permeates the institute, he easily gains the trust of other patients, like Ernie, who has an aversion to germs, Doris, who grew mentally fragile after a painful breakup, and Howie, among others. Each patient becomes convinced that Prot is genuinely from K-PAX. He proposes that he can take one individual with him when he departs, igniting hope among the patients, many of whom express a desire to escape Earth on a predetermined date—July 27 at 5:51 AM. When Howie seeks Prot's guidance, he is given three tasks aimed at confronting his fears. Howie successfully spots a rare blue bird, overcomes his germ phobia, and is ultimately prepared to embrace uncertainty.
As the anticipated date approaches, Mark confronts Prot about its significance, believing it ties back to traumas from Prot's past. Prot claims he can communicate with animals and offers insight into the nature of life on K-PAX. When Mark attempts regression hypnosis, he uncovers a startling truth: Prot may actually represent an alter ego of Robert Porter, a man with a tragic history involving the loss of his family, leading to a suicide attempt. The sheriff reveals that Robert killed the murderer of his wife and child, leaving Mark to grapple with this grim reality.
On that fateful July day, the hospital staff watches the camera feed in Prot's room go static at exactly the time he stated he would leave. They find Robert Porter, now in a catatonic state, suggesting that Prot's essence has departed. The patients, left perplexed, do not recognize Robert, who is whisked away. In the aftermath, Mark continues to care for the unresponsive Porter while reflecting on how Prot's encouragement has impacted the lives of the other patients. With newfound resolve, Mark embraces a deeper connection with his family, particularly reconciling with his estranged son, leading him to cherish the moments they have together.
> “We discover that our universe repeats itself and with it, our mistakes. Make this time count.” In the end, Prot's message resonates with Mark, pushing him to embark on a journey toward redemption and healing.
