At some unspecified point during the events of The Secret of NIMH 2, Timothy's older brother, Martin, ends up in the clutches of NIMH...having either been captured by Dr. Valentine, or having attempted to play the hero (out of an apparent jealousy of his younger brother).  As part of Dr. Valentine's hairbrained schemes, he has been altering animals through abusive electro-shock applications.  Until one day, during an careless moment, a gradually transformed Martin gets the drop on Dr. Valentine, and slips an electro shock device on his head, and shocks him.  This is the beginning of Martin's transformation into evil, and his increasing addiction to electro shock.  He takes over NIMH, forcing all of the human staff members to think they're dogs, and turning the other animals trapped at NIMH into a private army, for his plans to eventually march on and take over Thorn Valley.  All but "The Lost Six", the remaining mice who were thought, by Nicodemus and the rats of NIMH, to have perished in the air shafts of the NIMH ventilation system during the original escape. 

Evil Martin is a megalomaniac, bent on conquering Thorn Valley and making Jenny McBride his queen.  And he's apparently lost all brotherly love for Timothy.  The question is whether or not Timothy can save him and if the rats of NIMH can restore him to his old self.

Quote: "I suppose I have changed a bit...but for the better!"

Origins of the character: This character does not appear in Robert C. O'Brien's book "Mrs. Frisby And The Rats Of NIMH", or in the original The Secret of NIMH.  He is seen solely in the sequel.  This is perhaps one of the most blatant examples, in the misuse of an existing and beloved character from the book and the first movie, of how the sequel fails on so many levels.  And yet, in spite of it, this bizarre character corruption provides the impetus for some heroism on the party of Timothy, motivated by his filial love for his brother, and a desire to see him restored to normal.