Marie Hofer, a much respected 84 year old herbal doctor in Pennsylvania leaves her farm and goes into the forested mountains searching for new herbs to test for medicinal values. On Black Log Mountain, she forces her aged body to climb to a high, isolated rock ledge. A giant oak tree has been struck by a powerful lightening bolt and blasted into splinters. At the base of the tree, Marie discovers a mutant plant specie. She samples a tiny bit of the root to test for its effect on her body. She falls quickly into a deep sleep. Upon awakening, she feels unusually vigorous.
Over the next days as Marie tests the root, she discovers that she is growing ever younger, wrinkles fading, beauty returning.
Brent Dickson is falsely accused of the death of a young woman by Avery Stanholt, father of the young woman and the wealthy owner of an ocean shipping company. Stanholt hires the killer Laroche and his two gunmen to find and slay Dickson. Marie's friend Nell is murdered by Laroche. Marie sets out to solve the crime. Believing Marie is close to finding out he is the murderer, Laroche stalks her. She flees into the forest.
Marie, now young and with a young woman's desires, chances upon Brent Dickson who has assumed the identity of a dead friend and is working as a petroleum engineer for Shell Oil Company. She does not tell him her true age.
The killers discover Brent's location and come to kill him. They find Brent and Marie together. They wound Marie and murder Brent. Marie, filled with anguish at the death of her lover and burning to wreak vengeance upon the killers, traces them to the dangerous Boston waterfront. There she must devise a plan to trick her enemies into showing themselves so that she can slay them.
The conclusion of the story is startling, amazing.