Mrs. Brisby enters the rose bush and encounters an aggressive guard rat named Brutus, who chases her away. She is led back in by Mr. Ages, and is amazed to see the rats' use of electricity and other technology. She meets Justin, the friendly captain of the guard, Jenner, a ruthless, power-hungry rat opposed to Nicodemus, and finally Nicodemus himself. From Nicodemus, she learns that many years ago her husband, along with the rats and Mr. Ages, were part of a series of experiments at the National Institute of Mental Health (NIMH for short). The experiments boosted their intelligence, enabling them to escape, as well as extending their lifespans and slowing their aging process. However, they are unable to live only as rats, and need human technology to survive, which they have accomplished only by stealing. The rats have eventually settled on the decision to leave the farm and live independently. Nicodemus gives Mrs. Brisby an amulet, and tells her it will awaken when the wearer is courageous.