Twelve-year-old Ruth must spend her summer at the University of Toronto, where her mother is hired to clean the Institute of Biological Research. There, the lonely girl is befriended by Dr. Banting and his assistant Mr. Best, who are in search of a cure for diabetes. But much to Ruth's dismay, the research they are doing involves testing on animals. She's not the only one concerned about the dogs: a group of animal rights protestors become a regular fixture outside the institute.

When the group leader tries to enlist Ruth's support, she is torn between her sympathy for animals and her loyalty to Dr. Banting and Mr. Best.