This movie belongs very close to the top of the long list of 1980's comedy classic
It begins with Clark W. Griswold (Chevy Chase) and his son Rusty (Anthony Michael Hall) going to a car dealership to pick up the Antarctic Blue Super Sports Wagon they ordered. But the dealership has somehow gotten too many Metallic Peat Wagon Queen Family Trucksters, so they trick the Griswolds into buying one of those instead (by crushing their old car before they can take it back). "If you're making a drive across country," Clark later paraphrases the dealer to his wife Ellen (Beverly D'Angelo), "this is your automobile."
So they set off in the Family Truckster (after losing half their luggage before they even get out of the driveway). More crazy things than I can tell you about in one write-up happen along the way. They stop in Kansas to visit Ellen's cousin Eddie (Randy Quaid), the quintessential hick. His wife Catherine greets them, one baby inside, another in her arms, and several other children clustered around her. The oldest son, Dale, introduces Rusty to the world of nudie magazines, and the oldest daughter Vicki (Jane Krakowski!), champion pig raiser, proves to the Griswold daughter Audrey that farming is actually cool by presenting a shoebox full of marijuana.
But the family's fun is spoiled by Ellen and Eddie's Aunt Edna (Imogene Coca), a nasty old lady with an extremely ill-tempered dog. The Griswold are forced to drive them both to Phoenix to the home of a third cousin, Normie. They never do see cousin Normie, but that's another long story.
Several times along the ride, Clark is distracted by a girl in a red Ferrari (Christie Brinkley). Somehow she turns up at the same hotel as them one night, the same night that Clark and Ellen have had a fight. He goes to the bar, where he meets her, feeds her a big line of bull that she pretends to believe, and jumps into a freezing cold pool with her (then yells so loud that he wakes up the entire hotel). Ellen, offended that he thinks she doesn't know how to have fun, drags him back down there and jumps in, and then she screams loud enough to wake up the hotel.
At last they make it to Walley World, only to discover that it's been "closed for two weeks to clean and repair America's favorite family fun park. Sorry! (insert goofy laugh here)." And that's the last straw.
Clark drives back to town, buys a bibi gun at a sporting goods store, holds security guard John Candy hostage, and makes him take them on all the rides anyway. Park owner Roy Walley shows up with the SWAT Team (which, for some reason, in all movies starring the original cast of Saturday Night lIve, says "hut hut hut" all the time). After hearing their tale, he decides not to press charges.
The closing credits include further pictures of the trip - including the whole Griswold family wearing Walley World hats, sitting in the airplane they rode home.