Willard is a social misfit who is made fun of by his co-workers. When he is squeezed out of the company started by his deceased father, his only friends become a couple of rats he's been raising at home. However, when one of the rats is killed at work, Willard goes on a rampage--exacting revenge by using his rats to attack those who have been tormenting him. Led by the unusually intelligent, and deadly rat, Ben, an army of rats descends upon the office, committing a series of grisly murders.