Someone is out to prevent world peace at all cost. Whenever two countries seem to be making peace, some aircraft would drop some bombs at a political hotspot and peace talks are scuttled. French military aircraft have been stolen and used in these attacks. Biggles' French colleague, Marcel Brissac doesn't think this is the work of political extremists or terrorists. He suspects there is a highly organised and syndicate at work, back by unknown but wealthy industrialist and financiers, stoking up wars to sell arms at higher prices to both sides.
Marcel has a slender lead--one of the syndicate's pilots, named Voss, has been captured and he is a deserter from the French Foreign Legion. And many of the recent deserters of the Legion seem to have aviation backgrounds.
So "Biggs" and "Hepple" follow Marcel into the Foreign Legion under fake ex-R.A.F backgrounds and they soon find themselves posted to the Legion training centre at Zebrit, Algeria. There, the orderly sergeant, Voudron, approaches them and, hinting that he knows of their aviation backgrounds, entices them with the prospect of an alternative job with better pay and a chance to fly. He takes them to the "Villa Mimosa" to meet a local lawyer, Jules Raban who tells them they will be joining a private squadron whose aircraft are sometimes used to "pacify" tribesmen for industrial interests which work in undeveloped parts of the world. The pay is good but no questions must be asked. Raban introduces Biggles and Ginger to the pilot who will fly them to the squadron: none other than Voss! Voss has apparently been tried and got off on a light sentence and has still works for the syndicate.
Voss takes Biggles and Ginger to a nearby French Air Force base where they attempt to steal a Breguet light bomber. But the French authorities are more alert than Voss expects. There is a shoot-out and Voss is fatally wounded as their stolen aircraft is taking off. Biggles gains control of the damaged aircraft but is forced to crash land nearby.
Biggles and Ginger make their way back to Zebrit where they are punished for before out after curfew but Voudron is nonplussed. He sends them to the Villa Mimosa again to meet Raban. There Biggles and Ginger also meet Johann Klutz, another senior figure in the syndicate who appears to have come to Zebrit to learn what has gone wrong. Raban instructs Biggles and Ginger to proceed the next day by regular air service from Algiers to Alexandria to a rendezvous at a bar named the Aladdin's Lamp.
On the way back to camp in Raban's car, Biggles discovers Marcel's signet ring in the seat. Reasoning that he must have been kidnapped and might be in the Villa Mimosa, Biggles and Ginger make their way back to the house. There they meet Algy, who has been sent with Bertie by Air-Commodore Raymond as reinforcements. Joudrier, Marcel's boss, is also in Zebrit. Biggles fills them in on the details and suggests a raid of the Villa, but not until the next day to give Biggles and Ginger time to depart for Algiers.
The next day, Raban's car takes Biggles and Ginger to Algiers Airport where they find Klutz waiting for the same flight. Shortly before take-off, Algy and Bertie also join them, having rushed from Zebrit. They slip a note to Biggles telling him the raid was a success and Marcel is safe.
Arriving at Alexandria, Klutz sends Biggles and Ginger to the Continentale Hotel where Biggles and Ginger check in and then make their way to the Aladdin's Lamp bar. There they meet another pilot named Leffers, who tells them he will fly them out to the secret squadron the next day.
Returning to the hotel, Biggles and Ginger find the place dark and quiet. Making their way up to their rooms they stumble upon the body of Klutz. They catch sight of the assailant and subdue him after a brief scuffle. He tells them he is Cy Lindsay, an ex-member of the syndicate and now on a personal vendatta to wipe it out because he blames it for the death of his brother. Earlier on he had already killed Nestor Janescu, a rich industrialist who Marcel suspected of being one of the figures behind the syndicate. While Lindsay is talking to Biggles and Ginger, they are interrupted by Leffers and Lindsay shoots him as well.
Biggles decides it is time to pull out, and finding a compass course in Leffers' pockets, he and Ginger make their way to the airport rendezvous Leffers gave them. They tell the mechanic they are taking the aircraft alone as Leffers is dead and ignore the mechanic's protest that there is another passenger. They start up and get away just in time, for they see in the distance approaching, the other passenger--Von Stalhein! Apparently he has been recruited to head the operation in view of all the recent deaths. With Von Stalhein, Biggles also sees two others: Pantenelli and Festwolder, two other rich industrialists on Marcel's list of suspects.
Biggles and Ginger arrive at the location of the secret squadron in North Iraq, at a place called the Valley of the Tartars. They find the place in disarray. Klein, the chief pilot has been killed in a sandstorm. There is no fuel for the aircraft and no radio. With Klein gone, discipline has fallen apart. No guard has been posted and the men seem only interested in getting cigarettes. But they tell Biggles the syndicate still owns a Dakota and they hope that will come soon, with fuel and supplies.
Biggles and Ginger grab some guns from the unguarded armoury and go over to a nearby ruined castle some distance away. There they decide to settle down and await the arrival of the Dakota which they are sure will bring Von Stalhein, and trouble. As though that were not enough, from the vantage point of the castle, they see a Kurdish tribesman in the hills overlooking the squadron in the valley. A sign of further trouble, Biggles thinks, as he might be a scout for a raid on the squadron, now so ill-prepared to defend itself.
The Dakota arrives and Biggles sees Von Stalhein, Pantenelli and Festwolder approach the castle but Biggles stops them at the bridge to the gate, threatening them with his gun. Von Stalhein, is unable dislodge Biggles and decides to abandon the Valley, taking all the squadron away with him in the Dakota, leaving Biggles stranded. But outsiders now take a hand--in the form of the Kurdish raid Biggles has been expecting. In the ensuing pandemonium, Pantenelli, Festwolder and most of the squadron are killed. Von Stalhein and a few others take refuge in the castle where they agree to a truce with Biggles while they deal with the common threat.
The situation becomes desperate, with food and ammunition for those inside the castle running short. And they are not particularly encouraged to see a de Haviland Dragon circling over the valley. Ginger thinks it might be Algy, but Biggles reasons that even so, the pilot must surely conclude that no one in the secret squadron survived the Kurdish raid.
But the next day, Audaxes of the Iraqi Air Force arrive to bomb the Valley, putting the Kurds to flight. The Dragon follows and lands in the Valley, discharging not just Algy and Bertie but also Marcel and Air-Commodore Raymond, who gets introduced to Von Stalhein. It seems Algy really was in the Dragon the previous day, and, without much hope of finding survivors, they returned at the very least to find and bury the bodies.
Von Stalhein and the other squadron members are released in view of the difficulties of prosecuting their diplomatically complex crimes, while Biggles and co. depart for home, having done their part in destroying the syndicate