Three aircraft, all carrying V.I.P.s have disappeared trace over Africa, the most recent being an R.A.F. Hastings carrying an American army general. Biggles is not convinced that these are accidents. He reasons that it must be the work of a hostile power which is motivated not by the desire to kill but to abduct the V.I.P. for information. To do this, the enemy has to draw the aircraft off its course and then somehow force it to land.
Biggles convinces his superiors to act on his hypothesis and thus begins Operation Lex. Biggles sets off for Africa in another Hastings, having sent word out that another V.I.P. would be traveling home from Africa. Biggles and Ginger's Hastings, acting as a bait, takes off from Kano, Nigeria along the same route as the previous V.I.P. flight, shadowed by Algy and Bertie in a Halifax.
Somewhere over Liberia, Biggles notices his aircraft compass behaving strangely. Comparing it with a special compass insulated with lead, Biggles and Ginger find that they are being drawn westwards instead of northwards. Shortly thereafter, all four engines of the Hastings cut out and an unfamiliar twin-engined aircraft shows up which shepherds them to a jungle airstrip where they make a forced landing.
As the aircraft touches down, Biggles instructs Ginger to jump out unobserved. Biggles, on landing, is captured by a group of African soldiers who take him to a jungle camp to see their leader, a man who calls himself Christophe and who dreams of setting up his own African empire. Christophe is surprised to find that Biggles has no passengers and certainly no V.I.P. but he throws Biggles into a wired enclosure where he meets the other prisoners--the passengers and crew of the previous Hastings.
Among the prisoners is the American General Homer Mander, who tells Biggles that the U.S. had indeed been experimenting with a secret weapon, an electronic device to lure an aircraft off its course and also interfere with the ignition systems of its engines. This experiment had however been stolen by a mechanic named Dessalines, who turns out to be a Christophe's pilot and partner.
Shortly after, Von Stalhein, Biggle's old enemy arrives with Zorotov, another Eastern European secret service agent who Biggles had last met in Biggles in the Blue. It seems Christophe is bargaining with them, trying to sell the secret weapon and General Mander's secret papers for money which he needs to raise his African army.
Meanwhile Ginger, having seen what happened to Biggles, makes his way to a rendezvous with Algy and Bertie. Algy drops Bertie off and tells them he intends to return to Britain to fetch the Auster so that he can land more easily in the difficult terrain around them. He will return in a few days. In th emeantime, Ginger and Bertie are to go back and attempt to locate Biggles.
Ginger and Bertie make their way by night undetected to the perimeter of the camp outside the wired enclosure where the prisoners are kept. There they are in time to eavesdrop on Von Stalhein talking to Biggles. It is clear Von Stalhein is playing his own game, double crossing Christophe and also his partner Zorotov. Christophe has the secret weapon but refuses to turn it over. Vol Stalhein offers Biggles a pair of wire cutters as a means of escape. In return he wants Biggles to steal the aircraft with the secret weapon and fly him to a place of his choosing. Biggles refuses but Von Stalhein says he wants the other prisoners, especially General Mander, to have a say in the matter. He will give time for Biggles to consult them, and return in 30 minutes for their answer. The alternative, he says, is that Christophe will execute Biggles in the morning.
After Von Stalhein leaves, Ginger and Bertie make contact with Biggles and work out a desparate plan: on Von Stalhein's return, Biggles and General Mander will keep him occupied and demand to see his wire cutters. Bertie will threaten him with his pistol while Ginger makes a grab for the wire cutters. This plan, or at least the first part of it is smoothly executed and Ginger soon has the cutters but there is a problem when Zorotov is seen approaching the enclosure, apprarently suspicious of Von Stalhein. To prevent him from talking, Von Stalhein shoots him and tell runs off to raise the alarm. Nonetheless, the wires are cut and the prisoners escape. Biggles instructs Tony Wragg, the pilot of the V.I.P. flight to take the escapees away in Biggles' Hastings while Biggles, Ginger and Bertie remain behind to destroy the secret weapon.
The Hastings takes off but Dessalines follows soon after, flying the secret aircraft in pursuit. He returns later, having failed to intercept the Hastings but crashes his aircraft in a botched night landing. While Biggles is glad of the escape of the Hastings, he is nonetheless annoyed: the aircraft carrying the secret weapon is now lying in the open on the airstrip rather than in the hangar on the perimeter which is more easily reached unobserved.
Weighing their options, Biggles decides that since Christophe and Von Stalhein are now stranded without any aircraft, they must be prevented from calling in outside help. Leaving Ginger and Bertie to rest, he goes back alone to the camp where he sets fire to the radio hut. Returning, he is in time to hear a multi-engined aircraft starting up. It appears Tony Wragg's V.I.P. Hastings has been on the strip under concealment all along. They watch as Dessalines flies the Hastings off with Von Stalhein on board--leaving them to wonder what Von Stalhein's next move would be. The trio then make their way into the forest to get nearer to their rendezvous with Algy.
Biggles decides to split up--Bertie will proceed to the rendezvous while he and Ginger remain near the airstrip in case Von Stalhein returns. Bertie sets off but soon stumbles into a party of natives who capture him and march him back, exhausted and injured, in the direction of Christophe's camp. Fortunately, Biggles and Ginger spot the group and stage and ambush, scattering the natives and rescuing Bertie. They make their way back to the airstrip.
Back at the airstrip, Biggles and Ginger see that the secret aircraft has now been moved to the hanger under heavy guard by Christophe's men. Ginger suggests setting fire to the thatched roof of the hanger with an improvised bow and arrow. They try it but soon discover, as Biggles puts it wryly, the difference between movies and real life.
There is little time to rest. By daybreak Von Stalhein returns in the Hastings. As the aircraft rolls to a stop, a group of Europeans armed with automatic rifles emerge and promptly proceed to gun down all of Christophe's men at the airstrip. Biggles is aghast at this betrayal but the commotion gives Biggles just the opening he needs to approach the hangar undetected. Inside, he finds fuel drums stacked carelessly and now punctured and leaking, making it easy for him to set fire to the aircraft. The resulting smoke and flames also shield his escape from Von Stalhein's guns.
With nothing left of the secret weapon, Von Stalhein's men depart. Surveying the aftermath of the massacre, Biggles comes across Christophe, alive but wounded. He begs Biggles to take him to where he can get medical treatment. In exchange he offers his last trump card which he has held back from Von Stalhein--General Mander's portfolio of secret documents. Biggles accepts, and that effectively put an end to the conflict at the airstrip. On Algy's return, Biggles and friends fly off for British territory with Christophe while his private army scatters. Christophe, a U.S. citizen is handed over to the U.S. authorities to be "taken care of", thus putting an end to an extraordinary affair.