Mrs. Brisby is the main character in the children's book "Mrs. Frisby and the Rats of NIMH" by Robert C. O'Brien. She is a field mouse who is determined to save her family, especially her sick son, Timmy. Mrs. Brisby is brave and resourceful, facing various challenges in her quest to protect her home. In the story, Mrs. Brisby encounters a group of genetically modified rats who possess advanced intelligence due to experiments conducted at NIMH (National Institute of Mental Health). These rats, led by Nicodemus, help her navigate the dangers she faces, showcasing themes of courage, friendship, and the importance of community.