The cartune ostensibly stars Andy Panda (voice of Sara Berner) and his father, Papa Panda (voice of Mel Blanc), but it is Woody (voice of Blanc) who steals the show. The woodpecker constantly pesters the two pandas, apparently just for the fun of it. Andy, meanwhile, tries to sprinkle salt on Woody's tail in the belief that this will somehow capture the bird. To Woody's surprise, Andy's attempts prevail (comically, the mound of salt placed on Woody's tail is so heavy that he cannot run away), and in an ending very similar to 1938's Daffy Duck & Egghead, Woody is taken away to the funny farm—where his captors prove to be crazier than he is.
Like most of the early 1940s Lantz "cartunes", Knock Knock carried no director's credit. Lantz himself has claimed to have directed this cartoon (although it has been theorised that Alex Lovy was at least partly involved in directing the cartoon), which features animation by Lovy and Frank Tipper, a story by Ben Hardaway and Lowell Elliott, and music by Frank Marsales. Knock Knock was Marsales' final score for Lantz.
As the first appearance of Woody Woodpecker, Knock Knock is also the first cartune to feature Woody's trademark laugh, a gurgling cackle voice artist Mel Blanc had been perfecting since high school. This is also the laugh Blanc used for Happy Rabbit, a predecessor to Bugs Bunny in the 1939 cartoon Hare-um Scare-um. This cartune is also notable for featuring a very gruesome Woody design, something that was softened by 1942 and later changed into a much more realistic and more easier to animate woodpecker by 1944. This gruesome first Woody design featured Woody with red "vest feathers" (instead of white), buck teeth in some shots, thick ringed legs, a green tail, and a big chin which made him look more like a pelican than a woodpecker.
Woody's first words are his trademark "Guess who?" as he pops through the roof of Andy Panda's house, except the voice is normal-sounding instead of sped-up as Woody's voice normally would be.
When Knock Knock was first broadcast on The Woody Woodpecker Show, the ending containing the several screwball woodpeckers was removed. This scene was censored in very countries