The novel opens with Dr. Marina Singh reading a letter from Dr. Annick Swenson to Dr. Singh's boss and secret lover, Mr. Fox, CEO of the pharmaceutical company Vogel. The letter reports the death of Dr. Anders Eckman, Swenson's colleague at a drug research site in the Amazon rainforest. When Eckman's widow begs Dr. Singh to find out what happened, Mr. Fox agrees to send Dr. Singh to the Amazon. Mr. Fox's other motive is that Dr. Swenson was given a blank check to conduct research into a new miracle drug, and refuses to inform him of her progress.
Finding Dr. Swenson proves to be difficult. Dr. Singh flies to Manaus, Brazil, and finds that the only people who know Dr. Swenson's whereabouts are an Australian couple named Jackie and Barbara Bovender, who are tasked with hiding her whereabouts from the outside world. Eventually Dr. Swenson surprises Dr. Singh in Manaus, and they travel in a boat piloted by a young deaf boy named Easter to the rainforest research site, near the encampment of an indigenous people called the Lakashi tribe. The women of this tribe bear children until the end of their lives, an ability they gain from eating the bark of an endemic tree called the martin. The drug whose research Vogel is funding is one that will prevent or undo menopause and allow women to give birth throughout their lives. Over time, Dr. Singh discovers that, unknown to Vogel, the bark of the martin also serves as a vaccination against malaria; it is this drug that Dr. Swenson is primarily concerned with. She fears that no pharmaceutical company would fund such an unprofitable venture, so she uses secrecy to acquire the funds for her humanitarian project; also, she worries that the Lakashi people would be destroyed if the outside world discovered the potential of the martins. Dr. Singh learns that Dr. Swenson has become pregnant, at the age of 73, making herself the first human test subject for the fertility drug.
Mr. Fox eventually visits the research site with Mrs. Bovender and a local taxi driver named Milton. En route, their boat was attacked by a possibly cannibalistic tribe. During the fracas, Mrs. Bovender sees a white man among them who she thinks is her father. Dr. Singh allows Mr. Fox to leave without finding out the dual purpose of the drug he has been funding. Meanwhile, Dr. Swenson's fetus has died and she has Dr. Singh perform a Caesarian section on her; the baby is born still and with sirenomelia. Afterwards Dr. Swenson tells Dr. Singh of her suspicion that the man Mrs. Bovender saw was possibly Dr. Eckman, whose death she had never confirmed. Along with Easter, Dr. Singh sets out to rescue Dr. Eckman. When they find the tribe, Dr. Eckman is indeed living among them, but Dr. Singh discovers that the only way they will give him up is to exchange Easter for Dr. Eckman. Dr. Singh and Dr. Eckman return to camp without Easter; Dr. Swenson is outraged that the boy she cared for was left behind. Dr. Singh and Dr. Eckman return to Minnesota; Dr. Eckman rejoins his wife and sons, and Dr. Singh continues on home.
