Colonel Allen Faulkner (Richard Burton), a middle-aged British mercenary and former Royal Army officer, arrives in London from Switzerland to meet the rich and ruthless merchant banker Sir Edward Matherson (Stewart Granger) at his West London row-house. The latter proposes an operation to rescue Julius Limbani (Winston Ntshona), imprisoned former leader of the fictitious central African country of Zembalia, who is due to be killed by the military dictator who overthrew him. Limbani, whose people believe he is already dead, is being held in a remote prison, guarded by ferocious African troops known as the "Simbas", under General Ndofa.Faulkner provisionally accepts the assignment and sets about recruiting his officers, all of whom have worked with him on previous African operations. They comprise:Lt. Shaun Fynn (Roger Moore) is a former RAF pilot. He is working as a currency smuggler, but when he realizes that he's actually running drugs, he kills the mafia drug dealer and consequently has a death contract placed in his head. Matherson forces the crime boss to lift the contract at the last moment.Lt. Pieter Coetzee (Hardy Krüger), is a South African mercenary who only wants to return to his homeland and buy a farm, but can barely afford to pay his rent in London.Captain Rafer Janders (Richard Harris) is hired as an experienced mission planner. He initially refuses the job, as he's making some money as an art dealer and is planning a Christmas vacation with his son, Emile. But Faulkner persuades Janders to join the mission as the tactician.Retired R.S.M (Regimental Sergeant Major) Sandy Young (Jack Watson), is asked by Faulkner to serve as drill sergeant to train the troops and assist in recruitment. He is very willing, but his wife strongly disapproves.With the tacit approval and support of the British government, the 50 soldiers are transported to an unspecified African location, equipped and mercilessly trained by Young. The day before the mission is to begin, Janders exacts a promise from Faulkner to watch over his son Emile, and take care of him should Janders die on the mission. Faulkner agrees.The 50-man mercenary group are transported by plane and parachuted into the African country near Zembala Prison on Christmas Day. Coetzee uses a powerful crossbow with cyanide-tipped quarrels to take out the prison sentries. The rest of the guards are killed silently with cyanide gas. They rescue Limbani, but he is clearly a sick man and is later hit by crossfire during the battle. All of the Simba guards, along with one Cuban Army officer and advisor, are killed in the battle. The group, suffering no casualties, then commandeers several jeeps and other military vehicles from the prison and makes its way to a small airfield to await pickup.But backers of the project, led by Matherson, reach an agreement with the Zembalese government concerning valuable copper mining concessions, and the airplane due to collect them is recalled at the last minute.From this point onward, the abandoned mercenaries are forced to fight their way through hostile territory, pursued by the Simbas. It is also revealed here that Cuban, Soviet and East German army officers are training and advising the Simbas on their fighting skills and giving them orders to take out all of the foreign mercs.Meanwhile, the relationship between Limbani and Coetzee develops from initial animosity: "I bleed red like you, white man; don't call me kaffir" to one of understanding, as Coetzee comes to understand and appreciate Limbani's struggle, and realizes that white and black must work together.As they make their way south, heading for Limbani's home tribal land, they approach an old rickety wooden bridge. Only the first few vehicles make it across, when they come under aerial attack by a Simba fighter bomber aircraft that drops napalm. The bridge collapses under the weight of a fuel tanker, leaving the last jeep stranded on the northern river bank. In that jeep is Limbani, Coetzee, and medical orderly Arthur Whity (Kenneth Griffith).The mercenaries separate into two groups to continue to make their way to Limbani's home village, where they intend to provoke a revolution. However, most of the allied rebels are too ill-equipped and weak to fight. During all of this, Faulkner is forced to kill his own men who are gravely injured by the airstrike. Coetzee observes, "we can't leave them to the Simbas", whom will torture them to death.Coetzee is then killed while saving Limbani from a Simba ambush, leaving another soldier, a young Nigerian named Sgt. Jesse Link (John Kani), to carry Limbani. Witty is killed by machete-armed Simbas while trying to fend off another ambush, allowing the rest of the platoon to escape. Faulkner is reunited with the rest of the remaining 32 mercenaries at Limbani's village where an Irish missionary, named Father Geoghegen (Frank Finley), alerts them to the presence of an aging transport plane, a Douglas C-47 Skytrain aka "Dakota" at an old runway to provide for their escape.At the airfield, the mercs make their last stand in a climatic battle. As the masses of Simba troops attack, the group guns down dozens of attacking Simbas, while transporting the wounded Limbani on a stretcher to the plane, but many mercs are killed in the fierce battle, including the teenage soldier Esposito (Glyn Baker). Sandy Young is killed while he is running with Faulkner to board the airplane. Faulkner makes it on board, along with the tough gung-ho merc Tosh Donaldson (Ian Yule). Jesse Link also manages to safely get on board with several mercenaries carrying Limbani. While in the cockpit reading the plane for takeoff, Fynn is shot in his left leg, but manages to keep the plane moving. The last remaining mercs, including a wounded Jock McTaggart (Ronald Fraser), manage to board, but Janders is badly wounded after getting shot in his left side and can't get onto the moving plane; he implores Faulkner to shoot him as he is running down the runway with mobs of Simba troops in chase right behind him. Faulkner cannot bear to shoot his friend, but there is no hope. As Janders cries out his son's name: "Emile! Emile!", Faulkner reluctantly shoots and kills him just as the plane takes off.The plane with the wounded Limbani and the surviving mercenaries, numbering only 13 men out of the original 50, is initially refused landing permission in nearby Rhodesia, but after they provide proof that Limbani is aboard, they are given permission to land at Kariba. But by the time they land, virtually out of fuel, Limbani has died of his wounds. Flynn passes out, wounded but still alive despite blood loss. Seeing that Limbani has died, Faulkner regards the whole mission a failure.Three months later, having managed to return to London, Faulkner breaks into Sir Edward's residence, holds him at gunpoint and takes half of the originally agreed payment from his safe. He then kills him. Faulkner and Flynn make a quick getaway.In the final scene, Faulkner fulfills his promise to Janders; he visits Emile at his boarding school. Faulkner offers to tell Emile about his father.