The following version of the book was used to create this study guide:Heffley, Greg; Diary of a Wimpy Kid: The Deep End. ABRAMS Books, New York, NY, 2020. Kindle AZW file.

Greg Heffley and his family are living in Gramma's basement while their new house is being built, and the basement is beginning to feel really cramped. Greg's parents, his older brother Roderick, and his younger brother Manny are in the single room abiding by Gramma's rules, including that they stay out of sight when she has friends over. Greg is at the mercy of Manny's early bedtime and is sharing a futon bed with Roderick. They are often bored though Mom assures them they are benefiting from using their imaginations. Dad is working from home and the family is on a tight budget but Mom is determined they will do something fun as a family. They discover Greg's Uncle Gary has abandoned a motor home and that it provides the perfect opportunity to get away from Gramma's basement.

Their adventure begins with a trip to the store for camping supplies and the clerk's advice makes Greg uneasy. He tells them how to avoid bears and ticks, and sells them a snakebite kit, flare gun, and compass. By the time they check out, Greg is worried about all the things that might happen. They spend hours cleaning up the motorhome then set out in search of adventure. They have a few misadventures at first, including overcrowded campgrounds and taking a swim in a fish hatchery pond. At the next campground, they float down a river that also becomes very crowded and ends with Greg losing his swim trunks.

They stop next at a national forest where they are the only campers in sight. Greg briefly becomes lost but finds his way, then Dad scares them with a spooky story around the campfire. When a bear arrives, Manny scares it away with a flare gun. A park ranger then makes them leave the park, saying the flare could have started a forest fire. They consider calling it quits but find another campground, this one called Campers' Eden. There are lots of activities and amenities, including a pool, a lake, an arcade, and a camp store. However, there are also lots of other campers and the Heffley family winds up in the economy row at the back of the campground. Greg sets out to check out the place while Dad drains the sewage tank and sets up the motorhome. He soon discovers that everyone is running wild with no regard to rules, and no one is trying to enforce the rules. When Mom insists, Greg spends time playing with a group of boys about his age. They are having fun until they get in trouble for spraying soda around the snack bar, then Greg discovers the others are willing to tell on him to save themselves.

When a storm suddenly arrives, Greg rushes back to the motorhome where he discovers a skunk is inside with his family. They all stink that night and Greg goes to the store the next morning in search of supplies to eliminate the smell. He discovers everyone is panicking because the only bridge leading to the campground has washed out, meaning they are all stranded there. As campers panic and the situation becomes dangerous, the Heffleys decide to take their chances crossing the nearby stream. The storm has made the water too deep, however, and the motorhome washes downstream until Manny heroically turns the wheel, lodging the motorhome in place of the damaged bridge, allowing all the other campers to leave the campground. That's when Greg and his family discover that Campers' Eden really is a paradise campground, as long as they are the only campers.