An armored train is en route to the Denzali prison, carrying a fugitive. Meanwhile, Barney Ross (Sylvester Stallone) leads his team, The Expendables, in pursuit from a helicopter. Strategically, they set up a wire a few miles along the tracks. As Lee Christmas (Jason Statham), Gunner Jensen (Dolph Lundgren), and Toll Road (Randy Couture) initiate an attack on the guards, chaos ensues. Climbing onto the rooftop, they engage in fierce combat before Lee ventures inside the train to rescue the captive, known as Doctor Death (Wesley Snipes).
Upon liberating him, Lee attempts a quick escape. However, Doc becomes captivated by a portrait of a general, stalling their progress. Recovering their focus, they ascend the train's rooftop, only for Doc to make a shocking decision. He dashes to the train's front, eliminating guards with ruthless efficiency, ultimately steering the train straight into the prison, resulting in the obliteration of the general and the facility itself.
Doc, a former member of the Expendables, reveals the crimes that led to his imprisonment. Although he murdered numerous men, he was incarcerated for tax evasion—an ironic nod to Wesley Snipes's own legal troubles. Gunner hands Doc a knife so he can groom himself, while he heads to the cockpit to brief Barney. With urgency, they discuss locating the arms dealer Victor Min. To entice Doc into joining, Barney presents him with a bag filled with knives that once belonged to him, delighting the former Expendable.
The team lands in Somalia, where they reconvene with Hale Caesar (Terry Crews). Together, they navigate through the docks, engaging in combat with enemies. They spot a helicopter descending, from which Conrad Stonebanks (Mel Gibson) emerges. The team is taken aback to see him alive, presumed dead until this moment. In a frantic shootout, Stonebanks and his thugs retaliate, leading to a frenzied chase. Despite the team's efforts to evade the armed assailants, Stonebanks manages to escape by helicopter, injuring Caesar before unleashing a terrifying bomb that plunges our heroes into the river.
Despite rushing Caesar to the hospital, their attempts to aid him offer little reprieve. In a moment of reflection, Barney places his lucky ring beside Caesar and later finds solace in a bar, regrettably deciding that he must forsake his allies to protect them from further danger. However, Lee Christmas races to change his mind, but Barney’s determination remains unshaken.
Traveling to Las Vegas, Barney reconnects with an old ally, Bonaparte (Kelsey Grammer), aiming to recruit fresher, younger talent for the team. Once Bonaparte learns of Stonebanks's survival, he readily agrees to assist. Their search takes them to Wyoming, where they encounter Thorn (Glen Powell), notable for his impressive hacking skills and daring stunts. Next, they venture to a New York nightclub, where they meet Luna (Ronda Rousey), who impressively dispatches troublemakers. They also meet Galgo (Antonio Banderas), whose questionable background leads to a humorous rejection by Bonaparte. They continue their recruitment journey to an air base in California, discovering Mars (Victor Ortiz), a well-armed candidate, before locating John Smilee (Kellan Lutz) in Mexico, who initially refuses to listen to Barney's offer but later joins the mission.
With the new recruits assembled, Barney introduces them to the veterans before setting off to confront Stonebanks. Drummer informs them that the CIA desires Stonebanks alive for prosecution at The Hague. In the company of his fellow Expendables, Barney and the young recruits embark on their mission, learning about the war crimes connected to Stonebanks.
Thorn devises an intricate plan for infiltrating a location where Stonebanks is to meet an arms dealer. Their execution is precise; they swarm the warehouse, eliminating resistance until they successfully capture Stonebanks. The shocking revelation unfolds that he was a co-founder of The Expendables with Barney, once seeking wealth through illicit means. As tensions rise, Stonebanks taunts Barney into considering murder, confident that he would resist. Outside, his henchmen descend, launching grenades that unleash devastation, knocking Barney into a ravine while his team is nearly executed—though Stonebanks decides to spare them.
In an unfortunate turn of events, Barney reunites with Trench. He receives a video of the captured recruits, provoking a fierce desire to rescue them. Encouraged by the quick-witted Galgo, he gathers the remaining Expendables—Christmas, Road, Doc, and Gunner—ready to face the chaos that lies ahead.
Upon finding the hidden location, they execute a rapid rescue, but unknowingly trigger explosives wired throughout the building, setting off a countdown. Quick-thinking Thorn manages to halt the timer at just nine seconds before they engage Stonebanks in a climactic clash. A spectacular showdown unfolds, featuring Trench's aerial support alongside returning Expendable Yin Yang (Jet Li). Each member excels in combat, exhibiting their skills as they eliminate threats. Barney and Stonebanks finally face off in a brutal hand-to-hand conflict, where Barney ultimately prevails, shooting Stonebanks down for good.
As explosions erupt around them, Barney sprints towards the roof. In a breathtaking finale, he leaps to grab a hanging rope from a helicopter, escaping just in time as the building crumbles beneath him.
Eventually, Caesar recovers and reunites with the team at a bar. The four recruits earn official status as members of The Expendables, joined by Galgo's exuberant celebrations. Luna thanks Barney with a tender kiss for his dedication, and Smilee light-heartedly contemplates stepping into a leadership role, prompting Barney to humorously advise him to dream on. Meanwhile, Christmas and Doc engage in a friendly competition throwing knives, and the recruits cap off the night with a spirited karaoke performance of Neil Young's "Old Man," much to Barney's astounded amusement as he reflects on the remarkable journey they've all shared.
