What sets the action of "Dr. Heidegger's Experiment" in motion is the titular character's invitation to take part in his experiment in restoring youth to the aged. After the narrator introduces Mr. Medbourne, Colonel Killigrew, Mr. Gascoigne, and the Widow Wycherly—all of whom have become disreputable over the years—Dr. Heidegger reveals to them why they have been invited:

"My dear old friends," said Dr. Heidegger, motioning them to be seated, "I am desirous of your assistance in one of those little experiments with which I amuse myself here in my study."

The four guests have no idea what he is about to suggest or that it will directly involve them. The doctor has tried small experiments before, mostly the kind that seem like parlor tricks. However, this time he asks if they believe that a fifty-five-year-old rose could be brought into bloom again. When he demonstrates that it is possible, they dismiss it as a magic trick. But when Dr. Heidegger claims that a friend of his has discovered the Fountain of Youth, one of the men reveals his curiosity:

"Ahem!" said Colonel Killigrew, who believed not a word of the doctor's story; "and what may be the effect of this fluid on the human frame?"

Dr. Heidegger discloses that they are, if they wish, to be his test case, and urges them to consider where their lives had gone wrong. In restoring them to youth, he will offer them a chance to avoid the missteps that have created unhappiness in their old age. He is testing to see whether they bring their (alleged) wisdom with them back to youth. But instead of being cautious after their first sips, they cry:

Give us more of this wondrous water! . . . We are younger—but we are still too old! Quick—give us more!

Once their youth is restored, none of the four heeds Dr. Heidegger's counsel to be careful. The men grapple to get to the young and beautiful Wycherly, and in the scuffle, break the vessel that holds the water. Dr. Heidegger's final words are an indictment of their behavior:

. . . lo! the Water of Youth is all lavished on the ground. Well—I bemoan it not; for if the fountain gushed at my very doorstep, I would not stoop to bathe my lips in it—no, though its delirium were for years instead of moments. Such is the lesson ye have taught me!