This novel is about love and sacrifice, because as Mrs. Frisby adventures, she does so for Timothy, the sick child who needs medicine. But this means that she has to give things up in trades to try and earn the medicine. Plus, she is sacrificing her time, and each encounter with people is a kind of sacrifice as well, because she isn't just socializing—she is trying to accomplish a goal for Timothy's well being.
Another important aspect of the story is the passage of time, because the characters have schedules to keep, and there are clocks around them that help them know what time it is, especially at Owl's house, mixing the symbols of animal instinct and time. The point is that Mrs. Frisby is giving up her time for Timothy's well-being. The question of time is an important idea to consider, because Mrs. Frisby doesn't have infinite time. Her time is very valuable, because it is her real life.
Therefore, the sacrifice she makes for Timothy is to sacrifice her life, for some passages of time. Then, by collaborating with those people, she learns about a new threat. Instead of saving one person in her community, she has to help the whole community save itself from the ghastly fate of experimentation, and perhaps even death. The idea of freedom is associated with nature, because in labs, the rodents are certainly not as safe as they would be in fields where they belong, in wilderness.
