Pia is no ordinary teenage girl. The result of a century-long genetics project has made her immortal. Pia lives with her mother and a group of scientists and support staff in Little Cam — a secure compound deep in the Amazon jungle. Unfortunately for Pia, the genetic line that was supposed to produce an immortal mate for her ended in an "accident" 30 years earlier. To become a member of the Immortis team, which will engineer another Mr. Perfect, Pia must first pass a series of tests, called "Wickham tests," to prove that she is capable of prioritizing the best interests of the group above the well-being of the individual. Although Pia's abilities include superhuman intelligence, she struggles with the Wickham tests, as she hates to cause pain to any human or animal, even if it's for the greater good.
Pia's scientist caretakers (whom she refers to as aunts and uncles, because many are, in fact, her relatives) keep her insulated from any knowledge about the outside world. On her 17th birthday, a tree falls on the electric fence that separates Little Cam from the rest of the world. Pia and her pet jaguar, Alai, venture through the gap and into the jungle. She runs into Eio, a boy her own age who lives with the Ai'oans — a tribe of natives living near the compound. Eio's father is one of the scientists who lives and works at Little Cam, and his mother is an Ai'oan who passed away when he was a young child. Before Pia returns to Little Cam, Eio gives her a flower and asks her to meet him again soon.
Back in Little Cam, the head scientist, Uncle Paolo, is performing an experiment on the oldest immortal rat in the lab. While the Immortis serum (created by pairing the nectar of the elysia flower with a catalyst) can create immortal beings after five generations of injected test subjects, drinking the nectar itself kills any being that is not immortal. At first, Roosevelt, the immortal rat, shows no ill effects from drinking the nectar. But then he dies, as the others did before him.
Pia decides to secretly visit the Ai'oan village, where the medicine man Kapukiri prophesies that she has come to save them. She falls asleep in the village overnight. When she arrives back at Little Cam, the scientists have discovered the gap in the fence. With the help of a recently arrived scientist, Aunt Harriet, Pia manages to fool the others into thinking she never left. Aunt Harriet offers to help her sneak out in the future.
Little Cam is thrown into chaos when it's announced that they are expecting visitors from Corpus — the company that funds their research. With the help of Aunt Harriet, Pia leaves the compound and spends the day with Eio and his adopted sister, Ami. Pia and Ami are almost killed by an anaconda before Eio kills it with a single arrow through the head.
Pia returns to Little Cam to find that the visitors have arrived early, and her absence barely escapes detection. The visitors insist that Pia must take the final Wickham test early. She is given a week to decide to euthanize an ocelot kitten named Sneeze, who is a carrier of FIV (the feline form of AIDS). Uncertain about what to do, Pia puts it off and scales the electric fence and visits Eio. But the scientists are beginning to become suspicious about her absences. When one of the scientists, Uncle Antonio, shaves his beard, Pia notices the similarity between him and Eio, and realizes that he must be Eio's father. Uncle Antonio tries to convince Pia that Little Cam is evil (although he won't say why) and that she must escape to the city of Manaus with Eio. Pia and Eio are both attracted to each other, but Pia can't picture herself with a mortal who will eventually grow old while she stays young and perfect. She decides to stay in Little Cam and join the Immortis team.
However, Pia can't convince herself to euthanize Sneeze. Against Pia's will, her mother kills Sneeze and gives Pia the credit. Now part of the Immortis team, Pia journeys into the jungle with a team of scientists to retrieve a vial of elysia nectar from the grove of flowers. Eio secretly communicates with Pia, begging her to visit the Ai'oan village that night. She complies, and Kapukiri tells her a legend about the origins of the elysia flower. Pia realizes that the spores of the flower are in her tears. The catalyst, which the nectar must be mixed with to neutralize its lethality, is sacrificed human blood.
Unwilling to believe that the scientists who raised her are murderers, Pia returns to Little Cam where she is invited to administer the first dose of the serum to Uncle Paolo (who will be a great grandfather to her future mate). But then she realizes that Ami is the human about to be sacrificed to create the serum. In fact, the scientists have been killing off members of the Ai'oan community for almost a century in their quest to create immortal beings like Pia. The "accident" that occurred 30 years ago happened when two of the subjects, Alex and Marian, fled Little Cam in rebellion against these immoral practices. Alex and Marian died in their escape attempt, and the scientists also murdered many more of Pia's ancestors in the name of research.
Pia refuses to kill Ami and manages to help her escape. But when Eio comes to Little Cam to tell Pia he loves her, the scientists capture him and try to force Pia to inject Eio with the lethal elysia nectar to create the serum. Uncle Antonio bursts into the room at the last second and holds the scientists at gunpoint. The trio nearly escapes when Pia's biological father, Uncle Will, opens the compound gates for them. But when Uncle Paolo tries to convince them to stay, Pia's mother sneaks up and holds Uncle Antonio hostage with a syringe of elysia nectar. Uncle Antonio steps back, impaling himself on the needle and sacrificing himself so Pia and Eio can escape. Eio sustains a gunshot wound in the shoulder, and Pia leaves him hidden in the jungle while she proceeds to the Ai'oan village and convinces the villagers that they should not attack Little Cam. Instead, Pia and the villagers destroy all of the elysia plants and throw the flowers in the river so the scientists will have no reason to stay.
However, that is not enough. Pia's confidante, Aunt Harriet, told Uncle Paolo that Pia can grow new elysia plants from her tears. He tricks her into surrendering by lying and saying that they have captured Eio. Rather than submit to Uncle Paolo's plan to have her grow new elysia plants for the rest of her life, Pia finds one of the elysia flowers the villagers cast into the river and drinks the nectar just as Eio appears out of the jungle and fatally shoots Uncle Paolo with an arrow. The rest of the scientists leave at the villagers' arrow points, and Pia waits to die in Eio's arms.
Instead, she wakes up the next day in the Ai'oan village. Rather than killing her, the elysia nectar simply reversed the effects of immortality. Roosevelt the rat died of old age, but Pia is only 17. She realizes that she has a full — though mortal — life ahead of her. She and Eio kiss. Aunt Harriet, the only scientist left behind in Little Cam, lives with the Ai'oans for several months before returning to civilization. Pia chooses to live in the jungle with Eio and the rest of the Ai'oans.