Against a backdrop of vintage documentary footage and clips from the movie Mad Max (1979) and other archival footage of war, a narrator's voice (Harold Baigent) in the near future remembers the time of chaos after the collapse of the oil-based economy, and the apocalyptic world war that followed. The narrator relates how gangs of scavengers took over the roads, pillaging for fuel, and how ordinary men were broken in the decay: men like Max Rockatansky (Mel Gibson), the Road Warrior, who lost his wife and infant child to the gangs, and became an empty "shell of a man."
Max and his dog are being chased down an Australian Outback road, two cars and a motorbike pursuing Max's V8 Interceptor. Max deploys his booster and outruns his pursuers, wrecking the two cars. The motorbike riders pull up as Max is collecting fuel from one of the wrecked cars, and the bike's driver, Wez (Vernon Wells), pulls an arrow from his own arm, screaming defiance at Max. Wez rides off, leaving Max on his own. Searching the wrecked truck, Max finds the mutilated body of a dead child, and picks up the child's tiny music box, amused by the tune it plays, "Happy Birthday." Max drives off a short time later.
Max spots a gyrocopter by the side of the road. He approaches cautiously, but when he catches the snake guarding the 'copter, a man camouflaged with sand bursts from the ground, aiming a crossbow at him. The Gyro Captain (Bruce Spence) has been using the 'copter to attract the curious, and get their fuel once the snake has done its work. Max warns him the Interceptor's full tanks are booby-trapped. The Gyro Captain orders Max to disarm the bomb. When the Captain opens the passenger door, Max's dog attacks the Gyro Captain and the tables are turned. To save his own skin, the Gyro Captain offers to show Max an oil-refinery not far away, suggesting that the group that lives there will give Max all the fuel he needs. Max takes the man prisoner and drives to the refinery.
They arrive on a hill next to the refinery to find it under siege from a gang using all sorts of odd-looking vehicles, from motorbikes to hot-rods. The gang is under the leadership of a huge, masked man calling himself Lord Humungus (Kjell Nilsson). As he watches through binoculars, Max spots Wez arguing with Humungus.
Max, the dog and the Gyro Captain settle in for the night. The Gyro Captain pleads for his freedom, but Max reminds him the deal was Max wouldn't kill him. The three eat from a small can of dog food, the Captain getting the last morsels from it after Max and the dog.
In the morning four cars make a sortie from the refinery compound, heading in separate directions, with Humungus' people in pursuit. One car is intercepted close to their hide-out, and its driver and co-driver captured and tortured. Watching through binoculars, Max and the Gyro Captain see that one of the victims is a woman, who is raped and then viciously murdered. Max arrives at the scene after all except the rapist have left. He disposes of the rapist with a set of bolt cutters, and releases the badly wounded male victim, Nathan (David Downer), who offers Max as much gas as he likes if he takes him back to the compound.
Pulling up outside the compound, Max gets out and enters the compound at gunpoint, carrying Nathan over his shoulder. Max's car (with the dog) is brought inside by the compound's Mechanic (Steve J. Spears), as the leader of the refinery-people, Pappagallo (Michael Preston), interrogates Max about the fate of the other escapees. Max can tell him nothing, and points out he just wants his gas, as Nathan promised. However, Nathan dies and Pappagallo informs Max that whatever arrangement he had died with Nathan. Just then, Humungus' gang approaches, and the gate to the compound (an old school-bus, reinforced with armor plating) is closed, forcing the refinery people to let Max stay inside, cuffed to a pipe.
Humungus tells the refinery-people he intercepted all their escapees, and has learned from the captives that they were sent out to find a semi-tractor big enough to haul the fuel-filled tanker from the compound. He offers the refinery-people the chance to walk away free, if they leave the refinery, the tanker and all the fuel to him and his gang. He gives them one day to decide. While Humungus is making his speech, the Feral Kid (Emil Minty) who gets in and out of the compound through hidden tunnels, throws a razor-sharp boomerang at Humungus' gang, killing Wez's companion. Wez goes berserk, and wants to storm the compound immediately, but Humungus wrestles him down, telling him to bide his time - he'll get his revenge once the fuel is theirs. Humungus & his gang drive away.
The people of the compound are divided; some want to accept Humungus' offer while Pappagallo and others, among them the Warrior Woman (Virginia Hey), try to dissuade them. As the discussion becomes more heated, they forget about Max, who undoes his cuffs. The Feral Kid crawls out of his burrow next to Max, who plays the musical box to entertain him and - when the kid laughs with delight - carelessly throws the toy to him.
The discussion isn't going Pappagallo's way: most of the refinery people want to give up, a sticking point being that they have no vehicle that can pull the tanker, and that they're no match for Humungus' brutal gang anyway. Max offers to deliver a rig to haul the tanker, the one he found on the road while being chased by Wez, in return for his car and all the fuel he can carry. The deal is accepted.
In the dead of night, Max and the dog set off on foot, Max carrying fuel for the truck in jerrycans. Near Humungus' camp, he stumbles and the noise alerts the guards. The Feral Kid, who's followed Max, draws them off by imitating the call of a coyote.
In the morning, Max arrives at the hill where he left the Gyro Captain shackled to a log, only to find that that resourceful gentleman has set off across the wasteland, dragging the log. Max follows his trail and catches up with him. They arrive at the gyrocopter, finding a dead man who'd been killed by the Captain's snake. Max checks the man for supplies, finding a shell for his sawed-off shotgun (which had empty chambers the entire time). Max, dog and Gyro Captain fly to the truck where Max found the music box when he was chased by Wez. Max gets the truck started and throws the Gyro Captain the keys to the shackles, then drives off with his dog. The Gyro Captain calls "We're partners!" after them.
The gyrocopter follows in Max's wake as he heads off to break through the siege. Max takes Humungus' gang by surprise, and - aided by the Gyro Captain who drops snakes on the pursuers - makes it into the compound, although the truck suffers substantial damage. Humungus' gang are in hot pursuit, and Wez gets inside the compound by climbing aboard the truck. The gang now attacks the gate to get inside. In the confusion, no one except Max notices that Pappagallo is wounded while defending the gate. Max runs to take his place, using a flame thrower to prevent the enemy from breaching the gate. Wez brutally kills one of the compound crew and escapes to the outside. The compound people now greet Max as a friend and hero, wanting him to drive the truck for the escape run. Max just wants to get his fuel and go. The Gyro Captain lands in the compound, attracting excited attention. He immediately befriends a pretty young girl (Arkie Whiteley), persuading her to run away with him.
That night, Humungus, furious with the turn of events, crucifies and tortures the remaining captives on a hill outside the compound, and threatens that none of them will get out alive. While the compound is listening to his ranting, the Gyro Captain and the girl make for the 'copter, but at the last minute, the girl changes her mind - she won't leave her people. The Captain stays in the compound with her.
Meanwhile, Max is fueling up and getting ready to leave. Pappagallo tries to persuade him to drive the truck. Max refuses, and Pappagallo in his disappointment confronts him, calling him a coward, no better than the garbage outside. When Pappagallo asks Max about his history, wanting to know how he became so heartless, Max hits him. Returning to his work, Max finds the Feral Kid has hidden himself in the Interceptor, obviously expecting to tag along. Max evicts him without ceremony. As Max pulls out of the compound at daybreak, The Gyro Captain tells Max he's making a big mistake.
Humungus' gang sees Max driving away, and Wez - disobedient to his leader's will - sets off in pursuit with some companions in Humungus' vehicle. Max uses his booster tanks to escape, however, Wez does the same and overtakes Max. Wez hammers Max's windscreen with length of pipe and the Interceptor rolls off the road. Severely injured, Max barely has time to crawl out of the wreck with his dog and hide behind a rock, before Wez's companions scramble down to the wreck. Wez wants Max alive. One of Wez' men kills Max' dog, however, Max is spared from capture when the booby-trapped tank of the Interceptor explodes when opened by Toady, killing Wez's men. Wez pulls away, leaving Max for dead. At the compound, the Gyro Captain sees the smoke from the explosion, and fetches a semi-conscious Max back to the compound on the 'copter.
Max wakes up in the infirmary to find the Feral Kid bringing him his gear. Outside, Pappagallo is going over the escape plan with his people. The tanker, driven by Pappagallo, is to leave first, to draw off the attackers, allowing the rest of the people to escape in another direction in the school bus and other vehicles, to rendezvous with the tanker at nightfall. Max convinces them to let him drive the truck, pointing out that - despite his injuries - he's the best chance they've got. One of the women gives Max a small supply of shells for his shotgun.
They set off, Max driving the now-fortified truck, pulling the tanker which is guarded by the Warrior-Woman, the Mechanic and another warrior. At the last minute, the Feral Kid (supposed to go in the school-bus) jumps on the truck. The tanker smashes through Humungus' lines and heads off with Pappagallo in an escort vehicle, and the Gyro Captain providing aerial support.
The school-bus and the other vehicles quietly leave the compound without attracting interest, just as Pappagallo had predicted. Some of Humungus' people then enter the abandoned compound to take possession, but the compound has been rigged with explosives, and blows up only moments afterward.
Humungus has been keeping Wez chained after his disobedience, but now sets him free. They all chase the tanker, and one by one pick off the warriors defending it, Wez killing the Warrior Woman, and Humungus killing Pappagallo. Things are looking bad for Max, as the gang starts shooting out the tires. The Gyro Captain creates a diversion, dropping Molotov-cocktails from above. The 'copter is damaged and goes down, but Humungus has to stop to put out the fire in his vehicle before he can join the pursuit again.
Only Max and the Feral Kid remain to battle Wez and another gang-member, who have climbed aboard the truck and are attacking the cab. Max gets rid of Wez by slamming the brakes, throwing him over the front of the truck. The Kid helps Max fight the other guy, and then Max makes a U-turn, heading back the way he came. The Kid goes out on the bonnet to retrieve Max's shotgun shells, when Wez, who's been clinging to the front of the truck, suddenly pops up to grab the Kid. Max hauls the Kid back to safety, just as Humungus, speeding to catch up and realizing too late the truck has turned back, slams into the truck in a horrific head-on collision. Humungus and Wez are destroyed, and the truck goes off the road on its side in a spectacular wreck.
When Humungus' people arrive at the wreckage, they can see even from a distance that what's spilling from the broken tanker isn't gas - it's red dirt: the tank was only a decoy. A little later, Max climbs out of the truck, carrying the Kid. He stops by a tap, letting the red dirt run through his fingers. The Gyro Captain comes along, driving his copter on the road. Max and the Captain exchange wry smiles, Max shaking his head tiredly.
At sunset, the school-bus and the other vehicles head off, with the Feral Kid waving out the back of the bus at someone. The narrator's voice comes on, telling of their journey north, to safety, with the Gyro Captain as their new leader and that the gas was hidden in canisters in the convoy all along. The narrator is revealed as the Feral Kid grown to manhood, and the Road Warrior is seen standing alone by the side of the road - "the last we ever saw of him."