A rebellious stoner named Moondog lives life by his own rules.
Moondog is a constant stoner/party animal, but is also a well-regarded poet who is said to be writing a new novel. However, he prefers to live his carefree lifestyle without rules. After his rich wife Minnie dies, their daughter Heather freezes Moondog’s share of her estate until he cleans up his act and writes his book.
Moondog continues to get himself into shenanigans with people he meets until he is finally inspired to keep writing. He finishes his book of poems, wins a Pulitzer, and earns his share of the money, only to burn all of it on a boat.
The film starts in the Florida Keys where the writer known as Moondog (Matthew McConaughey) is walking around, completely stoned out of his mind, and comes across a cat, which he takes for himself. He ends up walking to a little club that is holding a show. The patrons go crazy as they all love Moondog and watch as he recites one of his poems, which they all enjoy.
Moondog is currently working on his book of poems, but he prefers to spend most of his days getting wasted and/or super baked, while also fooling around with as many women as he can. While most of the locals view him as some kind of legend, they all still think he needs to get his act together.
Moondog calls his wife Minnie (Isla Fisher), whose family’s wealth is what keeps Moondog afloat. She informs him that their daughter Heather (Stefanie LaVie Owen) is getting married that upcoming weekend. Being so out of it, Moondog practically forgets his daughter is old enough to be married, but he also isn’t a fan of her fiance, whom he refers to as “Limp Dick”. Moondog heads back to Minnie’s luxurious home in Miami to greet her and then go down on her while they talk about the wedding and Moondog’s book. Moondog later goes golfing with his agent Lewis (Jonah Hill), who, while very fond of Moondog, wishes he would focus more on the book rather than his frivolous lifestyle.
On the day of Heather’s wedding, Moondog is out having sex with a woman he just met when he gets a call from Minnie as the ceremony is about to start. Moondog speeds on up and makes it on time. Never one to miss an opportunity, he proceeds to go up and make a speech, while also turning his attention to Frank, AKA Limp Dick (Joshua Ritter), before grabbing Frank’s crotch to make sure he can satisfy Heather properly. Moondog’s best pal Lingerie (Snoop Dogg) officiates the ceremony. During the reception, Heather talks to Minnie about how Moondog nearly ruined everything. Moondog goes to smoke weed with Lingerie when the latter shows Moondog a strong strain that he’s been dealing. Later, Moondog spots Minnie and Lingerie kissing. He appears surprised but then goes off the continue smoking his joint before disappearing.
Moondog goes off to a karaoke bar to mess around with a couple on a date before he is found by Minnie. The two drink and have themselves a good time before deciding to drive home. Minnie (the one driving) swerves into the path of a truck, leading to an accident. Both are taken to the hospital. Moondog has minor injuries and manages to walk over to Minnie’s room. The two share a laugh before Minnie dies.
After Minnie’s funeral, Moondog is informed by a lawyer that her will left half her estate to Heather while the other half is for Moondog, but Heather has it frozen and in her control until Moondog proves he can be responsible and finish his novel. Heather makes it clear to her father that he needs to shape up and grow up. Moondog responds by taking the piss on Frank and later getting a bunch of homeless people to show up and trash Minnie’s mansion.
Moondog now faces jail time. The judge orders him to either undergo rehab for a year or go to jail. Although he likes to joke about the whole thing, the judge makes it extremely clear that his days of screwing around are over.
Moondog goes to the rehab facility where he meets another party animal named Flicker (Zac Efron). The two get to know each other before they break out of the facility and go around town to get high and hang out with random women. After a while, Moondog and Flicker part ways as Moondog decides to go back to Miami.
Moondog runs into an old friend called Captain Wack (Martin Lawrence), who proposes a business venture to Moondog in which they give tours of the outer Keyes to families. Moondog agrees, and they manage to scam one family into sailing out with them to supposedly swim with dolphins. The men horrify the family with their vulgar language before making it to the spot with the dolphins. Wack goes out to swim with them to prove they are safe, but the family sees that those are clearly shark fins. Before Wack realizes his mistake, a shark jumps out and bites off his foot. He also ends up splitting with Moondog.
Moondog reunites with Lingerie, who informs him that he is wanted by the law for breaking out. Moondog hides out for a while with Lingerie and hangs with Jimmy Buffett. During their time together, Lingerie confesses to Moondog about his affair with Minnie, but assures him that she always loved Moondog. He doesn’t seem to mind at all, and the two just discuss what sex with her was like. While hanging wtih Buffett, he and Lingerie discuss a dream to Moondog in which he has to dress in drag to get around undetected. He proceeds to do that for real.
Not long after, the police head toward Lingerie’s estate to arrest Moondog. They quickly set their escape plan in motion by getting Lingerie’s glaucoma-infected buddy to drive Moondog, along with a huge stash of Lingerie’s weed, to his personal plane to take him to Key West. On the plane, the driver (also the pilot) tells Moondog that he is basically almost entirely blind but that he can see by smoking a gigantic joint. Moondog is totally cool with that.
Moondog continues to do what he wants, but also starts to write more. He finally completes his book, which he titles “The Beach Bum.” The book is such a hit that Moondog wins a Pulitzer Prize. At the ceremony, Moondog is asked to read a poem. He picks a particularly graphic poem that was inspired by his love for Minnie, and everyone applauds it. Heather, now divorced, is also proud of her father and decides to let him have his money. Moondog asks the lawyer to let him have everything in cash.
Moondog throws one more party on a boat with many people in attendance. He takes the large stashes of cash on the boat…and proceeds to light the WHOLE thing on fire. The boat explodes, and flaming bills start raining down on the excited crowd. Meanwhile, Moondog sails away in a paddleboat laughing to himself.