The movie begins with Professor John Oldman (David Lee Smith) packing his belongings onto his truck preparing to move to a new home. His colleagues show up to give him an unexpected farewell party: Harry (John Billingsley, a biologist); Edith (Ellen Crawford, a fellow professor and devout Christian); Dan (Tony Todd, an anthropologist); and Sandy (Annika Peterson), a historian who is in love with John. As they are walking into the house, Edith spots an unknown Van Gogh painting of John's with the note "To my friend, Jacques Borne" on the back written in French. Once inside, his friends persistently ask John why he is leaving.
Another friend, Art Jenkins (William Katt), an archeologist, and his student Linda Murphy (Alexis Thorpe), arrive and John's friends continue to pressure him for the reason for his departure. John poses the question, "What if a man, from the Upper Paleolithic survived until the present day?" They assume that John is working on a science fiction story and play along with the discussion. As the film progresses, John slowly reveals that he is the "caveman" under discussion when he mentions that he was given a chance to sail with Christopher Columbus in the 15th century. The revelation starts off with John's recollection of the terrain of prehistoric Earth and John's origins, which happens to be roughly 14,000 years ago. John reveals that he was once Jacques Borne, a friend of Van Gogh's, and he is now "moving on" since some people have noticed his lack of aging.
As they take a break from the discussion, Art who is concerned about John's sanity telephones another friend, Dr. Will Gruber (Richard Riehle), an elderly psychologist, explaining the odd situation and asks him to come over right away. In the meantime, Sandy confesses to John that she loves him, but John tells her they can never be together because of his immortality. John resumes his story by stating that he was once a Sumerian for 2000 years, then a Babylonian under Hammurabi, and finally a disciple of Gautama Buddha. Dr. Gruber arrives, at which point Dan mentions that John's tale is as impossible to be disproved as it is to be verified, a response to the many attempts to poke holes in John's story by his colleagues. The discussion takes a turn into the biological and physical condition of John and the topic of death. Gruber propels the discussion deeper into the topic of death and tension rises as Gruber interrogates John on that subject. The tension culminates with Gruber pointing a gun at John. After the drama ends with the departure of Gruber, Harry reveals that Gruber's wife has passed away the day before and the profession of John's immortality had hit Gruber very hard. Art and Edith are upset with John's story, while Harry, Dan, Sandy, and Linda appear to be more sympathetic.
John also mentions that he is not a follower of a particular religion, and he doesn't believe in an omnipotent God. John's audience is shocked when he reveals how he survived the crucifixion when he was Jesus by "blocking the pain", a technique he learned in India. He explains the origin of the Resurrection, Moses, and other events and people in the Bible. Tempers and emotions rise as Edith pressures John to recant his story, which offends her deep-rooted faith in the Bible. Dr. Gruber returns to the scene and apologizes to John for his "infantile behavior". As John continues to pack his belongings onto the truck, his colleagues begin to talk about the possibility of John being mentally ill or high on drugs.
The discussions of John being Jesus, Christianity, and the Bible continue and Edith, unable to take John's tale anymore, breaks down crying. Emotions in the room run high. Gruber takes charge of the situation and sternly demands that John end his "high tale" and give closure to the story; he threatens John with the possibility of locking him up for observation. John apologizes to everyone and, as he doesn't want to further upset any of his friends, he tells them that all he just told them is just a story.
As each of John's friends leave, John apologizes to Harry and Edith, while Art and Linda leave without many parting words. When it is Dan's turn to say goodbye, it is hinted that Dan believes John's story. After everyone but Dr. Gruber and Sandy has left, Dr. Gruber overhears John relating to Sandy some of the other pseudonyms he has used over the years. One used over 60 years ago was the name of Gruber's father, a chemistry professor from Harvard, who had left the family. Gruber, shocked and over-excited, suffers a heart attack and dies. After Gruber's body is taken away, the movie ends with Sandy walking towards John (sitting in his truck); at the last minute he changes his mind to spend some part of his life with her.