When Nazi scientists discover his miraculous rejuvenation, they take a high interest in determining how this happened and place Dominic Matei (played by Tim Roth) into exile. While he is running to escape, Dominic reunites with his long-lost love and soon finds himself having to choose between the love of his life and his life's work researching the origin of language. Which will he choose?

The film is based on a novel by Mircea Eliade. Dominic begins his journey in his hometown of Piatra-Neamt, but finds that he cannot stay here. He travels to Romania with plans to kill himself with high doses of strychnine. When a lightning strike derails his plans and burns him horribly, Dominic is taken to a local hospital. There he starts his change from an old man to a youthful man, which the treating physician, Professor Stanciulescu, contributes to the jolt of lightning that he experienced. It is there that Dominic is presented with his "double," a much younger rendition of himself. Soon, though, he must leave the hospital with a false identity because of the untoward publicity that he and his "double" have received.

Dominic winds up in a clinic where Professor Stanciulescu takes notes about his life experiences to try and make sense of what has happened to him.

During their interview, the "double" reappears unexpectedly. To this point, Dominic thought that the "double" must have been a figment of his imagination. To prove that he is real, the "double" demonstrates a miracle with roses. Unknown to Dominic, a mysterious woman in room six of the clinic gives the Gestapo recordings of nightly conversations with Dominic and then disappears. For fear of being kidnapped, Dominic flees to Switzerland with false papers provided by the Professor.

In Switzerland Dominic meets up with his beloved Veronica, a woman with whom he spends much of the rest of his journey. They travel to India to complete some research and then to Malta. In Malta, Dominic discovers Veronica beginning to speak in different languages, but at first Veronica is not aware that she is doing this. Each time Veronica goes into these episodes, she becomes weaker, ages more, and her language becomes more ancient.

Dominic's "double" urges him to let her continue having these episodes, despite her suffering, so that the origin of language can be finally discovered. Because of his undying love for Veronica, Dominic cannot find it in his heart to do this. He believes that if he leaves the scene Veronica may regain her strength and youthfulness, so Dominic departs and returns back home to Piatra-Neamt.

While in his room resting, Dominic pulls out a picture of Veronica. She is youthful again and has two children. Now he knows that leaving her was the right thing to do. Dominic confronts his "double," trying to explain the difference between good and evil. The "double" insists that Dominic should have let Veronica go on to the origin of language, and that all of his research is wasted because he left her side.

Dominic ultimately shatters the mirror that brings forth his "double" to be rid of him completely. Within a short period of time, Dominic abruptly ages and dies. A rose, given by his love, appears in his outstretched hand.