Plot
Some time after the events of the previous film, Eddie and his family are no longer living on the bomb test site in Nevada. However, the government has given Eddie a job at the Chicago Atomic Testing Agency measuring his brain waves for radioactivity. With the dirt farm foreclosed on and the RV repossessed, Audrey Griswold has taken in Eddie and his wife Catherine, their dog, Snots, and precocious youngest son, Clark III (called "Third" for short), while the rest of their young children are staying with their grandmother. When a chimpanzee named Roy outperforms Eddie in every cognitive test, the agency, facing budget cuts, fires Eddie right before Christmas. Upon finding out the news, Eddie insults Roy, which results in the chimpanzee biting him.
Although the thought never occurs to Eddie, his boss, Professor Doornitz, is concerned that Eddie will sue the agency. As a preemptive countermeasure, Doornitz offers Eddie and his family an all-expense paid Christmas vacation to an archipelago nation in the South Pacific called Maluka. Eddie, Catherine and Third are joined on the vacation by their cantankerous and perverted uncle Nick, whose wife, Jessica, has left him for a younger man. Audrey also joins them, not wanting to spend Christmas alone after being dumped by her her boyfriend, Daniel. Third is not excited for the vacation and is embarrassed by Eddie's inability to hold down a job.
The group is welcomed to Maluka by their guide, one of the employees of the South Pacific Division of the Atomic Testing Agency, Muka Luka Miki. After a Christmas cookout, the group boards a boat to tour the neighboring islands. However, when Eddie is attempting to fish off the boat, he accidentally catches a large shark, which drags the boat far out to sea. In the chaos, Nick crashes the boat into a coral reef off the coast of an isolated island. Shipwrecked on the apparently deserted island, the group immediately decides that they need to find a food source. Everyone is upset that they are shipwrecked on a desolate island and cannot celebrate a true Christmas.
Determined to impress his son and prove that he is a man by providing for his family, Eddie starts a fire and goes hunting with Snots. Eddie chases a baby pig through the jungle, only to be confronted by an aggressive wild boar. Eddie manages to climb a tree, only to fall out of the tree and drive his homemade spear into the boar, accidentally killing it. Eddie brings the boar's carcass back to the campsite and they roast it for Christmas dinner. Catherine and the others are deeply impressed that Eddie killed with his bare hands to provide for his family, and Third tells Eddie that he is proud of him for the first time. On Christmas morning, the group exchanges a variety of homemade presents or regifted items, and they dedicate a grass hut built by Third and Eddie, which Eddie claims will be the first home they have that the bank won't foreclose on. However, as soon as they shut the door on the hut, it collapses.
While looking for help, Eddie signals a sea plane that lands near the island. The pilot, Melbourne Jack, tells them that the island is actually called King Halihalki Island, and he owns a resort ten miles away, so they could have simply walked to safety on the other side of the island. While preparing to fly them back to Maluka, Jack hits his head and suffers a severe concussion. Everyone boards the plane and Jack takes off, but he passes out from his head injury. With Jack incapacitated, Eddie successfully lands the plane in the water with help from the Maluka tower, saving his family. Aunt Jessica arrives in Maluka to see Nick, having had a change of heart. Professor Doornitz and Roy the chimpanzee arrive on Maluka to inform Eddie that he has heard of his heroics and is giving him his job back. Eddie declines, however, saying that he has gotten a new job. Eddie is revealed to be a pilot on Maluka, with Roy serving as his co-pilot.
