The title is a play on the comic strip character "Prince Valiant."
Yosemite Sam (here, a Viking named "Sam the Terrible") is rowing up a river towards a castle, and he is noticed by two people on a nearby riverbank, who retreat to the castle sensing an invasion. As he passes by Bugs Bunny's hole, Bugs peeks out and thinks that Sam's outfit is that of an "electric can opener broken loose". As Bugs sees Sam enter the castle, Bugs takes it upon himself to fight Sam.