Jenner was the chief antagonist of the 1982 animated adaptation of Mrs Brisby And The Rats Of NIMH, known commonly as The Secret Of NIMH, he is a rat who betrayed his own kind due to his desire to remain in the thorn bush rather than leave to a better life in the promised Thorn Valley.
He schemed against the kindly old leader of the rats, known as Nicodemus, with his assistant - Sullivan and ultimately came up with a plan to take get rid of Nicodemus and take control (although Sullivan was reluctant to harm Nicodemus).
During the moving of Mrs Brisby's home Jenner and Sullivan waited ready to cut the rope and crush Nicodemus under the falling rubble, however Sullivan falters and is pushed aside by Jenner who cuts the rope personally - killing Nicodemus.
Jenner then tries to use the situation to get the other rats to his cause via making it out as a terrible accident, however when Mrs Brisby interrupts with warning about NIMH arriving Jenner grows enraged and attacks her - forcing Justin to come to her aid and the two engage in a sword-fight, Sullivan ends up mortally wounded when he refuses to aid Jenner's madness and ends up delivering the fatal blow to the malicious rat via tossing a dagger at him, which strikes him in the back and kills him almost instantly.