From the official website of the film, now out on DVD:
Every 30 years, in the summer there is a mysterious unidentified creature claimed to inhabit Astaire's freshwater lake in Minnesota.
Along with Loch Ness monster Nessie and Lake Champlain's Champ, this monster is one of the best-known mysteries of crypto zoology. Most scientists and other experts find current evidence supporting the creature's existence unpersuasive and regard the occasional sightings as hoaxes or misidentification of known creatures or natural phenomena. Minnesotans believe in this legend, even though their theories may vary. The creature thought to be a plesiosaur being the most popular of these theories. But unlike Loch Ness or Lake Champlain, there is no canal linking Serpent Lake to the sea raising the question," Where does this creature come from? And during this time some people in or around the lake come up missing.
Who doesn't love a monster that haunts Minnesota? I hope it likes mashed potato pizza and pickled herring, because that's what it's going to taste when it chomps on our heroes.
I utterly adore the unapologetic Z-grade production values here. Such brio! And the characters are hilarious. The whole thing gives me a warm, Godmonster of Indian Flats kind of feeling.