The Louds rent a boat for a relaxing day trip on the lake, but the family ends up marooned on a desert island.
Lincoln explains that for the very first time in forever, everyone is free this Saturday. As luck would have it, today also happens to be fishing season, so the family has opted to spend their day by taking a boat trip around Lake Eddy. As the family gets ready, Lola asks Lana for her tire pump so she can inflate her floatie, Lana eats Rita and Lynn Sr.'s bait for breakfast, Lynn exercises her butt in hopes of creating a big splash when she cannonballs, and Lincoln convinces Lucy to join him on the hunt of a mysterious creature lurking in the lake named Plessy.
At Lake Eddy, the family rents their boat from Flip, who tells them to bring in back within three hours, or else he'll double their rental fee. As the family relaxes on the boat, the engine suddenly dies out. It turns out that the engine is filled with junk and leaking. Rita tells her family to get the oars, but the oars on the boat are merely just painted on. To make matters worse, there is nothing else that can be used to move the boat. Suddenly, the family ends up getting caught and swept down into the depths of a raging whirlpool.
With the boat completely destroyed, the family ends up being washed up on a deserted island. As the kids begin panicking that they're marooned and that their hopes of contacting someone to help them are gone, Rita and Lynn Sr. tell them to not give up so soon, saying that while they wait for the coast guard to come and pick them up, they can make the most of what they can find on the island. As the family ventures around the island, Rita, Lana, and Lynn find wood to start a fire, Lynn Sr. and Lisa find fresh fruit, Lincoln, Lori, and Lucy follow tracks that lead to fresh water and Luna creates a giant "SOS" in the sand. At night, the parents express their happiness at how optimistic everyone has been.
The next morning, the family wakes up and discovers that wild animals have arrived and eaten all of their food. As they chase the animals away, strong hurricane winds come in and destroy the housing and beds. As Lola chases after the bed she has made, she suddenly gets caught in quicksand. Lana tries to help her, but she gets stuck as well. The family manages to rescue the twins by lassoing them out thanks to a rope that Leni had. At that moment, the family hears a ship horn bellowing in the distance and discover that it's the coast guard. Unfortunately, the boat happens to be going away from the island, since it turns out that the "SOS" lettering that Luna wrote was washed away by the tide. With all of their hard work destroyed, and Lisa informing them that the coast guard only surveys the lake during fishing season (which only occurs every eight months), Rita and Lynn Sr. break down, declaring that they're giving up. However, the kids, motivated by all the perseverance they were given, tell them to not give up just yet, and state that they may have missed the coast guard, but they can still create their own boat to leave.
Sometime later, the family creates their raft and starts to sail off. Unfortunately, not long after they leave the island, a tidal wave flips them over, but this doesn't stop them, and proceed to paddle back to shore. After several hours of paddling, a dinghy arrives, and it happens to be Flip, who was looking for them after going over their rental limit. When Lana expresses that the boat was in poor condition and that it's a miracle that they're still alive, Flip decides to not fine them double and offers to let the family travel along. As they ride back home, Plessy, the lake monster Lincoln and Lucy were looking for, appears behind the boat and licks Lily. Lily tries to tell Lincoln about Plessy, but Lincoln misinterprets Lily's actions as going back to the island, and comments that home is the other way, much to Lily's annoyance.