The film starts with Lieutenant Mitch Buchannon (Dwayne Johnson) arriving at work to perform his duties as a lifeguard on the beaches of Emerald Bay, Florida. He sees a man parasailing on the water, but the man hits his head on a rock, forcing Mitch to spring into action. He dives into the ocean and swims to the man's rescue. Following this, Mitch goes for his morning run around the Bay. He passes through Sergeant Ellerbee (Yahya Abdul-Mateen II), a local policeman and beach patrol officer with whom he frequently messes with regarding their status as enforcers of the law around that area. While continuing his run, Mitch spots a small boy picking up a pouch of the drug Flaca on the beach. Mitch takes it from the boy and observes it. Mitch's fellow lifeguards are Stephanie Holden (Ilfenesh Hadera), who Mitch considers the best after himself, and CJ Parker (Kelly Rohrbach), who is the object of affection for aspiring lifeguard Ronnie Greenbaum (Jon Bass).
Ronnie is flying a toy craft on the beach with his colleague Dave (Hannibal Buress) when CJ runs up (in slow-motion, of course) to wish Ronnie good luck on the upcoming qualifiers, but Ronnie can barely put a sentence together around her. Arriving on the bay is Matt Brody (Zac Efron) on his motorcycle. He tries to introduce himself to one of the prospective lifeguards, Summer Quinn (Alexandra Daddario), but she ignores him. Brody then meets Mitch and Stephanie, explaining his new position on the team and his status as a two-time Olympic gold medal winner, but neither of them care.
Mitch tells Brody he has to earn his job, and Brody tells him off. Meanwhile, Victoria Leeds (Priyanka Chopra), the new owner of the beach's Huntley Club, is meeting with Councilman Rodriguez (Oscar Nunez) over a real estate deal. In exchange for his services, Leeds gives him a new watch that her men wear. Ronnie is on the beach with Dave eating baby carrots when he sees CJ again.
He starts to choke, leading CJ to perform the Heimlich on him. Ronnie spits out the carrot but he also has an erection. Before CJ can see it, Ronnie jumps onto a wooden lawn chair and gets his whole groin stuck between the boards. Things get worse when Mitch comes over to help, as observes Ronnie's got both his "beef and biscuits" stuck in there. Other beachgoers show up and start filming him.
When Mitch opts to pull it out like a tooth, Ronnie tries running but instead falls face-first, breaking the chair and getting unstuck. Mitch rounds up all the new hopeful lifeguards to begin their test in the obstacle course. Brody sits it out as he feels he doesn't have to do it. Summer gives it her all and passes through the course, though Ronnie struggles at first due to being heavier than the others. Still, he refuses to give up and keeps going, which Mitch takes notice of. By the end of the day, Mitch announces Summer, Ronnie, and Brody as his new trainees. Mitch puts Brody through his own specialized course, which involves ridiculous tasks such as lifting large truck tires and hauling refrigerators like weights.
Nearby on the pier, two young children fall into the water and their mother jumps in after them. Mitch, Steph, and CJ head in to rescue them while Brody follows and tries to be a hero. He goes to rescue the mother but she keeps smacking him. Mitch and CJ rescue the kids and Steph rides them to safety. Brody just ends up looking foolish. Mitch goes to Captain Thorpe (Rob Huebel) to complain about Brody.
Thorpe tells Mitch that Brody is a well-known skilled swimmer, but he lost it all when he puked in the pool during a race after a night of hard partying. His position on the team is part of a community service deal, as well as a ploy to make sure the Bay doesn't lose its funding, so Mitch has to work with Brody whether he likes it or not. At night, Summer and CJ find Brody sleeping under the pier. CJ takes him to find a place to crash, making him think she is taking him to Steph's home, but he is really going to be staying with Mitch. Mitch lets him stay and makes him sleep on a couch next to a fish tank with a mini-figure of Mitch "watching" him. Brody begins training under Mitch across the beach. He nearly steps on a poisonous sea urchin until Mitch points it out. On the water, Rodriguez tells Leeds that he made arrangements with his people for the real estate deal, but because he did not deliver on time, Leeds has her two thugs Frankie (Amin Joseph) and Leon (Jack Kesy) come around to intimidate Rodriguez.
He tries to call Leeds's bluff by saying he knows about her pushing drugs. She leaves her men to deal with Rodriguez. As the guys continue their training, Leeds drives up and has Mitch introduce her. She invites the whole team to an open house that night at the Huntley. Not long after she leaves, Steph answers a call about a yacht burning up on the beach. The whole team, including the trainees, rush out there to find the burning boat. Mitch hops on the boat and rescues three women but also finds an unconscious man. Brody dives under the water to try and help, but he gets caught under the fire.
Steph uses her jet ski to splash water around the fire while Summer pulls Brody out. When they get back to the shore, they find that the man they pulled out is dead, and it's Rodriguez. Ellerbee shows up, and Mitch tries to talk about what they've investigated, but Ellerbee tells him to stay out of what is a police matter. We see that Dave is working for Leeds as her tech guy.
He attempts to blackmail her so that he doesn't say anything about Rodriguez's murder. She has something else in mind. The team sits down to have lunch and discuss the fact that a dead body has turned up, in addition to finding more Flaca on the women they rescued (they were high off the stuff). They decide to attend the open house to get any more information on Leeds that they can get. That evening, the team goes "undercover" at the open house party. They assign Brody to be lookout to have eyes on Leeds. He does manage to get Leeds alone but he doesn't get any information because he's more focused on getting drunk.
Ronnie distracts Leeds while the others go to work by doing a crazy dance that gets CJ's attention. Mitch then goes to confront Brody over his failure at his task. Brody curses Mitch out and tells him he's just a lifeguard. Mitch humiliates Brody further by scaring him into falling into a pool, where he pukes again and everyone sees. Brody plays it off by telling everyone do shots, but he passes out. Meanwhile, Frankie and Leon take Dave's dead body on a boat out to the sea and slather him with meat chunks as they dump him in there to be shark food.
The next morning, Mitch tells Brody that he is great at what he does when he's on his own, but when it comes to working as part of a team, he fails to do so because he is selfish and only cares about himself. Brody begs for a second chance and tells Mitch and Summer that he noticed Rodriguez wearing the watch that Leeds's goons wear. The trio head to the morgue to continue their investigation by disguising themselves as hospital staff. In the morgue, they find Rodriguez's corpse and autopsy files. They see that there was no smoke in his lungs, which means he died before the boat caught fire. Mitch orders Brody to look under Rodriguez's scrotum for anything, but it's just an opportunity by Mitch to further humiliate Brody by getting pics of Brody looking at a dead guy's taint.
Summer sees Frankie and Leon approaching. The three hide in the drawers, but Brody ends up hiding on top of a dead old lady. To make things worse, he has fat from a corpse dripping on his face. The goons then open up Rodriguez's drawer and swap out an autopsy file to make it look like he did die from the fire. Brody then freaks out when the fat drips into his mouth, so he jumps out and gets hand sanitizer on his mouth. They are further exposed when Summer's phone rings.
Leon grabs the phone and runs out with Frankie. Leon tosses Summer's phone to Frankie. Mitch goes after Frankie while Brody and Summer try to catch Leon. Mitch chases Frankie on a jet ski and finds him trying to break into a family's home. The two fight in a little girl's bedroom after Frankie breaks the phone until Mitch puts a diaper bin on Frankie's head and knocks him out the window into the pool. Mitch, Brody, and Summer all meet with Sergeant Ellerbee as he angrily tells them to stop getting involved in the case. Captain Thorpe also shows up as the three try explaining that they did have evidence against Leeds's men, but all the evidence has been destroyed. Thorpe threatens to fire Mitch if he continues his investigation.
Frankie gets away with everything after complaining to the police over Mitch's actions. Mitch observes the Huntley getting their fish from another company, which he thinks is how they are smuggling Flaca onto the beach. He and Brody go undercover and sneak into the Huntley's kitchen where they find Leeds's men handling drugs. Mitch calls Ellerbee to report, but Ellerbee is tied up with having found Dave's body being partially torn up by sharks. Ellerbee also has Thorpe listen to Mitch calling. Thorpe calls Mitch into his office to fire him. He appoints Brody to be the new lieutenant, but Mitch tells Thorpe that the position should go to Stephanie since she knows the beach just as well as he does.
Thorpe doesn't care for Mitch's opinion and dismisses him. Although Brody tries to decline the offer, Thorpe threatens him with a bad record for his community service. The others aren't happy with Brody taking the position, despite his attempts to defend himself. Brody starts to handle things on the beach despite not having everyone's support. He catches a punk on the beach stealing people's stuff with a cooler that pulls things in. He tackles the thief and scares him away. When he opens the cooler, Brody finds a bag of Flaca. He then asks Steph and CJ to help give back everyone's things.
Brody goes to Ellerbee's office to give him a chocolate smoothie but also to use the cooler to take a file off his desk without him knowing. Mitch takes a new job at a phone store and cuts off connection with the rest of his team. When he looks ready to give up, he is approached by his old mentor... the original Mitch (David Hasselhoff). He inspires Mitch to keep up his work and to not give up on himself or his people. Brody approaches Summer with the file, showing Dave's corpse.
Ronnie sees the pics and recognizes Dave. He tells the others that Dave mentioned working with Leeds. Ronnie later shows Brody how he can hack into Leeds's servers, leading them to discover Leeds's ultimate plan - she wants to privatize the beach so that she can eliminate competition and spread drugs around undetected. The guys tell the ladies about this plan and they come together to stop Leeds. The team infiltrates Leeds's yacht during a party. Steph, CJ, and Ronnie find Murray Chen (Clem Cheung), a real estate guy that Leeds kidnapped as part of her scheme. Meanwhile, Brody swims under the boat and finds that Leeds has been smuggling drugs in the hull. Brody resurfaces to tell Summer to get in touch with Brody.
When he gets back on the boat, he gets knocked out by Frankie. The goons take Brody on another boat and lock him in a cage. Before she can kill Brody, she tells him that she bribed Thorpe to set Mitch up so he would get fired and get him off her back. She pushes the cage into the water, which Ronnie and CJ see. Just as Brody appears to accept his fate, he is pulled out of the cage. He sees Summer reviving him, but it's really Mitch, to Brody's disgust. Leeds is preparing to get away on a helicopter.
Mitch tells Ronnie to work the fireworks display to throw the helicopter pilot's sight off. Ronnie panics at first but gets confidence after CJ kisses him. As the fireworks go off, Mitch and Brody fight Frankie and Leon again. Frankie gets knocked into the water while Leeds shoots Leon dead for his incompetence. The guys go after her but she shoots Mitch in the shoulder and he falls into the water. He gets back up as Leeds holds Brody at gunpoint. Meanwhile, Ronnie and CJ find a big red button. Ronnie flips the switch, and Mitch notices the lights near him activating.
Brody hits Leeds's gun, making her run out of bullets as Mitch grabs a Roman candle, says a one-liner, and waits for Ronnie to hit the button to launch the firework at Leeds, obliterating her. Mitch then reveals he got up by using a sea urchin to boost his adrenaline, but he is in need of medical attention. Back on the shore, Mitch gets treated, and Ellerbee shows up to apologize to Mitch for doubting his claims. Thorpe shows up to try and berate Mitch, but Brody punches him in the face, and Ellerbee arrests Thorpe and Frankie.
Later on, Ronnie and CJ hook up, and Brody and Summer appear to start a relationship as well. They join Mitch and Stephanie as Mitch officially instates the trainees as lifeguards on the Bay. He then introduces them to their new captain, Casey Jean (Pamela Anderson), who enters in slow-mo. The film ends with the team doing their own slow-mo run on the beach, only for Brody to tumble hard on the sand. A series of bloopers and outtakes play during the closing credits, ending with both Mitches sitting on the beach as Hasselhoff-Mitch asks Rock-Mitch if he can be in the sequel..