English scientist and amateur artist Julian Karell attends the unveiling of his portrait of Eve Brandon, with whom he has fallen in love, at her father Sir Humphrey's home. When Julian is introduced to the Brandons' elderly guest Lady Minerva Aldergate, to her amazement he describes a romantic encounter she had with a man she presumes was his grandfather. After the painting is unveiled, one of Humphrey's guests notices that the style exactly matches that of a painting he saw fifty years earlier in Breslau, Germany. Julian proposes to Eve, but tells her their marriage will have to wait until his upcoming experiment is completed. When he returns home, Julian is disappointed to learn that a visit from renowned endocrinologist and surgeon Kurt Van Bruecken will be delayed. Julian then records in a journal labeled "Vita Immortalis" that as was expected, his "symptoms" have returned again after ten years. Late that night, Julian rescues Allen Guthrie, a young medical student, from attempted suicide, and the next day, shares with him Kurt's plan, written in 1885, for transferring glands from one human to another to prolong life. Guthrie agrees to take part in the experiment after Julian promises that if he survives, he can participate in the research. When Kurt finally arrives, the two men reminisce about their youth. Kurt urges Julian to make the results of his experiments public, but Julian, who is in his nineties, jealously guards their secret, which has made him appear to be a man of thirty-five, and claims that he has not yet perfected the technique. Julian is forced to seek another surgeon to perform his operation after seeing that Kurt's hands now tremble violently, but no physician he contacts agrees to the experiment, and Julian's face and hands slowly start to show signs of age. When Eve and Julian announce their engagement to her father, his friend and physician, Dr. Henry Latimer, probes Julian about his work and his connection with Kurt. Latimer is suspicious of Julian, especially after learning of his fruitless search for a surgeon, and he and Humphrey consult with Inspector Garth at Scotland Yard. Guthrie, meanwhile, has come to realize that he is being drugged and held prisoner, and after breaking out of his room and reading Julian's medical journal, he learns that all of Julian's previous subjects died. Later, Latimer goes to Julian's, ostensibly to meet Kurt, but Julian tells him that he is ill, and Latimer leaves after slyly getting Julian's fingerprints on his cane. A fingerprint expert at Scotland Yard later informs Garth, Humphrey and Latimer that Julian's prints exactly match those of an 1800s German murderer named Kornis, who killed a medical student every ten years. When Latimer and Humphrey take Eve's portrait to Scotland Yard, they reveal to her their doubts about Julian's character. Kurt urges Julian to bring Latimer in on the experiment, but Julian refuses to "share the glory" and shows no concern that Guthrie may die if he takes him to France, where a friend of his could operate. Now realizing that Julian is no longer the humanist he once knew, Kurt insists that he drop Eve so that she will not be hurt. Later, after Julian confesses to Eve the nature of his experiments, Guthrie dies from the operation. Although a taxicab driver identifies Julian as the man who took Guthrie home with him, Julian claims that Guthrie left the next morning and refuses to take Garth's bait and admit any connection to Kornis. After Guthrie's body is pulled from the Thames, Julian avoids returning home and urges Kurt to meet him on the train bound for Paris. When the police arrive at his house, Kurt seems addled and can only say that "no man is greater than God." Garth then learns that the portrait of Eve was definitely painted by the same man who painted the Breslau picture, and Humphrey calls to tell him that Eve has eloped with Julian. In their compartment on the train, Julian, meanwhile, shows signs of severe fatigue and starts to age dramatically. Upon hearing Julian confess that his work has made him more and more lonely through the years, Eve pledges that she will always be with him in spirit, and without showing his face, Julian rushes off the train at the next stop. When the police board the train searching for Julian, no one but Eve recognizes the ancient man who collapses and dies on the platform.