Who died in Mrs.Frisby and the rats of NIMH?

In "Mrs. Frisby and the Rats of NIMH," Mrs. Frisby's husband, Jonathan Frisby, has died before the events of the story. His death is a significant part of the backstory, as it is revealed that he was a brave and noble mouse who sacrificed himself to protect his family. His loss impacts Mrs. Frisby and motivates her actions throughout the story as she seeks to protect her sick son, Timothy.