Mrs. Brisby (Elizabeth Hartman) is a timid widowed field mouse that lives with her children in a brick on a farm. She and her family are planning to move out of the field in which they live because it is time for the field to be ploughed. However her youngest son Timothy (Ian Fried) has fallen dangerously ill and cannot be moved. Mrs. Brisby seeks advice from the Great Owl, who tells her that the mysterious Rats of NIMH might be able to help her. When she goes to the rats to ask for help, she meets their wise leader Nicodemus (Derek Jacobi) and finds out that both the rats and her late husband were part of some experiments at a place called NIMH. Mrs. Brisby is amazed by the rats’ elaborate home with elegant rooms, elevators and even electric lighting. The rats are unhappy with their dependence on humans and want to leave the farm to live independently. However, in order for the rats to move successfully, someone must put a bell on the farm cat, Dragon, so the rats know if he is nearby. In return for help with Timothy, Mrs. Brisby volunteers.