He is a scientist who performs cruel and illegal experiments on animals in his quest for genetic perfection.
An explosion in his lab gives Underdog his powers and mutilates part of Dr. BarSinister's head, driving him completely to the brink of insanity.
After several failed attempts, he manages to kidnap Underdog, and uses one of his invention to create a pill that contain a concentrated distillment of Underdog's powers. Dr. BarSinister and his hench-dogs swallow the pills, becoming superpowered beings themselves. Dr. BarSinister uses his newfound powers to attack the mayor's office, planting a bomb in the city hall in the process. Underdog manages to redeem the hench-dogs, throw the bomb out of range of the city, and defeat Dr. BarSinister.
At the end, Dr. BarSinister is arrested, and is implied that the effects of the pills wore off in his struggle with Underdog.