Boy meets rat.
Boy loses rat.
Boy gets rat back and turns it loose on his enemies.
For film buffs, this was one Elsa Lanchester's last movie roles, one of Sondra Locke's first and one of Ernest Borgnine's most delicious, om nom nom nom.
The director was Daniel Mann, who also made such classics as Come Back, Little Sheba (1952), The Teahouse of the August Moon (1956), Butterfield 8 (1960), and Our Man Flint (1966).
For trivia buffs, "Ben" (the sequel to "Willard"), featured "Ben" (the song) performed by Lee Montgomery and by Michael Jackson over the closing credits. This soulful ballad to a Rattus norvegicus won a Golden Globe for Best Song, was nominated for an Academy Award for Best Original Song in 1973 and became Michael Jackson's first U.S. #1 solo hit.
According to Ernest Borgnine's autobiography, they offered him a choice of a higher salary or a percentage of the box office. Borgnine chose a higher salary. That turned out to be a mistake, as Willard became the first "lines around the block" horror movie to go what we would now call "viral." It wound up grossing over 19 million 1971 dollars for, get this, Bing Crosby Productions.