Themes include the threat of modern war to human survival, the moral responsibility of scientists, and the importance of both individual freedom and sympathetic instincts.

12-year-old Allison Adam lives with her mother and 2 brothers in an isolated farmhouse by Seneca Lake (a reservoir that was being planned at the time O'Brien wrote his story) near Washington D.C. Every day after school, Allie stops by the walled grounds of Villa Petrograd, property of the Soviet embassy, and climbs a tree to view monkeys at a small zoo there.

Unknown to her, the embassy holds the secret research lab of Helmuth Schutz, a former Nazi now developing bioweapons for the Soviets—specifically, mutations of the bacteria coryna and anthracis.

Meanwhile, the eminent biochemist Fergus O'Neil is recruited by the U.S. State Department, to study stolen Soviet reports from Schutz's Group 17, and investigate his activities at Villa Petrograd, by posing as a vacationer.

After taking water samples one day, O'Neil rescues John Adam from nearly drowning, while testing the police's theory about his sister's bike accident.

Through research, he learns of Schutz's interest in a native Peruvian society that had lived in a hidden valley of the Andes, but died out after their water supply was contaminated with a mutagen.

The story finishes with a brief epilogue that explains O'Neil's new job analyzing "Water X", a German expedition's "accidental" destruction of Incan ruins, and Schutz's comfortable life in Germany, where he has a new identity as a professor and his biochemical research is progressing well.

From the gate Allison Adam enters, the hill is to the left, and the road to the right continues parallel to the estate's wall until it ends in a marshy area that borders the Potomac River.

The Little Seneca Reservoir in Montgomery County was created by one of two dams built in the 1980s to provide backup water sources for times of low flow in the Potomac.