It's 1963 and Katie Winston, a young zoologist working on her doctoral thesis at the Zoologico de Sao Paulo, is asked to write a brief impact study for a dam about to be built on a tributary of the Amazon River in the jungles of northern Brazil.
She travels from a bustling city to a construction camp at the edge of a deep jungle valley. After a harrowing adventure, just getting to the location, her arrival is met with animosity. The foreman considers her study a waste of time and her presence in a camp full of men, a dangerous distraction.
She befriends two heavy machinery operators. One is a giant of a man who played football for the University of Alabama and sees Katie as a surrogate daughter. The other, younger operator, Chris, shares an instant attraction with Katie. The rest of the workers are far less kind.
From the moment she arrives, it's clear she isn't expected to do any actual research or even write a report. A determined Katie decides to descend into the jungle valley alone. Everyone is taken by surprise, including the extraordinary primate society that call the valley home.
When she returns, she takes a stand. Protecting the valley and creatures becomes paramount, given their intelligence and important place in human evolution. Her discovery is met with hostility. Nothing will prevent the construction plans from moving forward, so Katie returns to the valley hoping her presence will keep it from being destroyed.
Unfortunately for Katie, the government is prepared to sacrifice her to keep the project on schedule. Trouble in Vietnam has prompted the development of a new explosive defoliant, and the valley will be the location of its first real world live fire test. The total destruction of everything in the valley will render the inconvenient discovery of the new species moot. Katie, cut off from any communication, has no way of knowing the seriousness of her situation.
With the clock ticking, Chris races to help Katie save the creatures.