Biggles is handed yet another of his secret diplomatic assignments--to fly a Professor Zorlan to a remote sheikdom on the eastern borders of Turkey, perform a few more side trips as directed, and fly him back, all with no questions asked. There should not be any danger, reassures Air-Commodore Raymond, but as usual, should anything go wrong, the authorities will not be able to help. For this mission, Biggles is given a new aircraft, a "Merlin", which Raymond describes as "the last word in air transportation".
Taking Bertie and Ginger as spare pilots, Biggles takes the Merlin into Asia Minor. Transiting in Ankara, a Colonel Osman Alfondari, claiming to be from Turkish Intelligence insists on coming along, claiming to be their escort. Not wanting to offend the Turkish authorities, Zorlan and Biggles reluctantly agree but Zorlan warns Biggles that he must not be allowed to follow and see what Zorlan does at their destination.
The Merlin arrives uneventfully at Quarda, an archaeological site in the tiny sheikdom of Zarat but things begin to go wrong almost immediately. Alfondari insists on following Zorlan when he goes to meet his contacts and Biggles has to restrain the colonel. Then Ginger discovers Alfondari trying to send a message on a portable radio which Biggles swiftly disables. Beginning to suspect that Alfondari is an imposter and a spy, they debate what to do with him--Zorlan wants him shot but Biggles will not act so ruthlessly on a whim.
Towards evening, a Russian MIG fighter approaches the area and performs a reconnaissance but fails to spot them as Biggles had camouflaged the Merlin. The next morning, however, things move more quickly. First, an aircraft of the Turkish Intelligence Service arrives and an officer approaches Biggles to take Alfondari away, saying that he might be an imposter. Biggles allows the Turkish officer to take Alfondari, or rather, he does not have much choice in the matter. Later, however, he wonders if he has been had. Still he is glad to be rid of Alfondari.
Later in the day, three MIGs arrive and begin to strafe the area. Again, the Merlin is not spotted and not seriously damaged. However the attack happens just as Zorlan is meeting his contacts, the Sheikh of Zarat and his retinue, in the ruins of Quarda. Fortunately, they are not hurt but, as their horses had bolted, Zorlan asks Biggles to flew his guests to the nearby oasis of Suwara, where the Sheik maintains a military outpost. After dropping the Sheikh's party there, Zorlan next asks Biggles to fly him to Rasal Al Sharab, another small sultanate about a hundred miles to the south, on the Iran-Iraqi border.
Along the way, the Merlin is almost bounced by a MIG and Biggles narrowly evades its attack by flying into a sandstorm. Forced down on a wadi by the extreme weather, the occupants of the Merlin pass an uncomfortable night before proceeding to their destination.
At Rasal Al Sharab, another passenger joins the Merlin and here Biggles learns a little more about the mission. The newcomer is a princess, the daughter of the Sultan of Rasal Al Sharab, who is to be married to the Sheikh of Zarat to seal a pact of alliance. Biggles must now take the Princess to Suwara, but as he nears the location, he spots red rockets, a signal that it is not safe to land there. Biggles now diverts to Quarda to await developments.
The Sheikh arrives at Quarda from Suwara and takes Biggles aside to speak privately. He tells Biggles has been preoccupied putting down a rebellion in his capital and at Suwara. In the meantime, he wants to consult Biggles on another matter. Part of Zorlan's mission has been to seek his signature on documents granting British interests a concession to drill for oil in Zarat. These papers had been in order, but there was also a separate sheet of paper granting Zorlan 7% of oil revenues. Is this legitimate? Biggles immediately suspects something is wrong and tells the Sheikh not to sign the paper. He will send Ginger and Bertie to Athens where they can contact the Air-Commodore directly for confirmation. The Sheikh agrees. He needs to depart but promises to return as soon as possible. In the meantime, he leaves some of his guards with the Princess and puts Biggles in charge at Quarda.
That evening, While awaiting the return of Bertie and Ginger, Zorlan approaches Biggles. He tells Biggles that the Sheikh is still holding out on some vital signatures. Zorlan offers Biggles a hundred pounds if he will help persuade the Sheikh to sign the papers. Nothing doing, Biggles tells him. Zorlan next suggests that he could force the Sheikh's hand by using the Princess as a hostage--a thought which gives Biggles another uneasy night.
Ginger and Bertie return the next morning from Athens. As expected, Zorlan's document granting him 7% of the oil proceeds is a fake. Biggles seizes Zorlan's documents, burns the fake paper and disarms him. However they do not have time to relax. Almost immediately they hear the sound of multiple aircraft engines. A Russian transport and it is dropping a dozen paratroopers! Ginger hurries off in the Merlin, to ferry the princess to safety at Suwara while Biggles, Bertie and the Sheikh's men hold off the paratroopers. The "Battle of Quarda" takes the better part of the day and things get desparate. Biggles finds himself down to three men because of casualties. Zorlan also gets himself shot when he tries to change sides, waving a white flag. The battle settles into a stalemate in the afternoon but Ginger comes back with the Sheikh and reinforcements, forcing the paratroopers to retreat and evacuate in their troop transport. Among the troops fleeing, they think they see Alfondari, who appears to have survived the battle.
With the tension at an end, Biggles ferries the Sheikh back to his capital. There the Air Police are treated to a sumptious meal before returning to London with the precious documents.