The overall theme of "Dr. Heidegger's Experiment" includes people's lack of willingness to reform, even from bad habits. It also explores how susceptible people are to the powers of suggestion. While the overall themes of the story seem overtly cynical of human nature, it can also be taken as a warning to avoid the pitfalls found in the overall themes.
The story begins with Dr. Heidegger inviting four of his old friends to participate in an experiment in his dusty old study. All of his friends are old, white haired and going through difficult times in their lives.
Each of the friends has a particular vice or character trait that has been the main cause of his current misfortunes. One was too greedy, for example, while another's hedonistic lifestyle caused him to suffer from many illnesses.
The doctor tells his guests he has found the secret to eternal youth, demonstrating by placing a withered rose in a vase, which instantly becomes restored to full bloom. All the guests are eager to try some of this elixir from the fountain of youth, but before they do, Heidegger suggests to them that they should use their new found youth to serve as an example to the younger generation, having experienced the pitfalls of life themselves.
With their new youth, all of Heidegger's friends inevitably repeat the same behavior that caused them problems in the first place.