After being admonished by his father, Dr. Ned Trescott, for damaging a peony while playing in his family's yard, young Jimmie Trescott visits his family's coachman, Henry Johnson. Henry, who is described as "a very handsome negro", "known to be a light, a weight, and an eminence in the suburb of the town", is friendly toward Jimmie. Later that evening Henry dresses smartly and saunters through town—inciting catcalls from friends and ridicule from the local white men—on his way to call on the young Bella Farragut, who is extremely taken with him.

That same evening, a large crowd gathers in the park to hear a band play. Suddenly, the nearby factory whistle blows to alert the townspeople of a fire in the second district of the town; men gather hose-carts and head toward the blaze that is quickly spreading throughout Dr. Trescott's house. Mrs. Trescott is saved by a neighbor, but cannot locate Jimmie, who is trapped inside. Henry appears from the crowd and rushes into the house in search of the boy, finding him unharmed in his bedroom. Unable to retreat the way he came, Henry carries Jimmie, wrapped in a blanket, to the doctor's laboratory and the hidden stairway that leads outside. He discovers the fire has blocked this way out as well and collapses beside Dr. Trescott's desk. A row of nearby jars shatters from the heat, spilling molten chemicals upon Henry's upturned face.

Dr. Trescott returns home to find his house ablaze; after he is told by his hysterical wife that Jimmie is still inside, he rushes into the house by way of the laboratory's hidden passageway. He finds Jimmie still wrapped in the blanket and carries him outside. Hearing that Henry is inside the house, Dr. Trescott attempts to re-enter, but is held back. Another man goes into the house and returns with the badly burned "thing" that used to be Henry Johnson. The injured men and boy are taken to Judge Denning Hagenthorpe's house across the street to be treated, but while it is thought that Dr. Trescott and Jimmie will survive their injuries, Henry is pronounced as good as dead; he is mourned as a hero by the town.