"Alfred Hitchcock Presents" The Hidden Thing (1956)

When a young man sees his fiancée struck and killed by a hit-and-run driver, he is inconsolable. When questioned by the police, he is unable to remember anything that he saw, even though he wants very much to have the driver caught. A week later, he is visited by a stranger who claims he can help. The visitor tells him that his own son died in the same way, and he offers to help the young man to remember the license plate number that he saw, by using special memory recall methods. It's not clear, though, what the visitor's motive for helping might be.