Jonathan Brisby was an Uplifted Mouse, one of the NIMH Escapees, who lived most of his life on the Fitzgibbon Farm in the Brisby Burrow, while maintaining close ties with the Rats of NIMH in the Rosebush Colony.
In the beginning, Nicodemus was a street mouse, no different from other mice, aside from a higher-than-average sensitivity to the Power. One day, he, along with several other rodents, was captured on behalf of the scientific organization NIMH, and given injections which both prolonged his life and improved his mental capacity while also improving his psychic abilities, which uplifted him.Nicodemus figured out how to open the test subjects' cages, he deferred leadership to Jonathan, who led the mice and rats out of the NIMH building through the ventilation system. On the way, all the mice were nearly blown away by a ventilation accident. However, Jonathan and Jacob Ages were able to cling on to a rope and climb back to safety. Later, Jonathan was also the one to figure out how to open a locked door on the roof, at great risk to his own life; this event in particular earned him the Rats of NIMH's eternal gratitude.