Chapter 1: Conference at the Yard[]
Air Commodore Raymond congratulates Biggles on his promotion to Air Detective Inspector and almost immediately assigns him a difficult mission. A noted desert explorer, Doctor Guthram Darnley has discovered an unusual plant named gurra at Wadi al Arwat in the Arabian Empty Quarter. Not just a botanical curiosity, the resin of the plant also makes a powerful narcotic 100 times more powerful than marijuana.
Chapter 2: An Assignment in Arabia[]
Although Darnley had told no one about his discovery, cigarettes spiked with gurra have started appearing in London. Somehow someone has obtained a sample and has exploiting it on a commercial scale. Since the gurra plant is only known to grow in that one wadi, Raymond wants Biggles to fly out and burn it.
Chapter 3: A Riddle in the Sands[]
Biggles, Ginger and Bertie set out in two Percival Proctor aircraft. After transiting at Aden, they reach Wadi al Arwat where they discover, to their surprise, that the gurra shrubs have already been burnt! Nearby they find Kuatim and Zahar, two coastal Arabs who had accompanied Darnley on his expedition when he had discovered the gurra. Kuatim is dead but Biggles manages to revive Zahar with water and food.
Zahar tells Biggles how Abu bin Hamud, the third Arab, had guessed the secret of the gurra and had persuaded him and Kuatim to go together to Wadi al Arwat to collect the seeds and resin. Then Abu bin Hamud had set fire to the plants and made off with their camels, leaving him and Kuatim to die. Biggles flies Zahar back to Aden.
Chapter 4: Biggles Makes a Call[]
Air Commodore Raymond had recommended Captain Jerry Norman, a retired Political Officer, as a useful local contact in Aden. Biggles is anxious to know the whereabouts of Abu bin Hamed as he is the only person with gurra seeds. Norman had asked around and no one seemed to know where he is but a contact reported that Abu had been seen meeting with one Nicolo Ambrimos, an extremely weathy incense merchant who seemed to have an interest in any merchandise. There were even rumours that he dealt in hashish.
Biggles decides to call on Ambrimos. As a pretext, he plans to organise an oil prospecting expedition and wants to employ Abu bin Hamed and hopes Ambrimos could locate him. To his surprise, or perhaps not, Ambrimos has a reputation of having spies everywhere, and already "knows" Biggles is prospecting for oil and operates aircraft. He claims not to have seen Abu since the Darnley expedition--which of course Biggles knows is a lie.
Biggles decides to fire a shot in the dark and suggest that Ambrimos has a faulty intelligence service as Zahar had been seen in Aden recently. This works, and Ambrimos is visibly surprised.
Chapter 5: Knotty Problems[]
After leaving Ambrimos' luxurious villa, Biggles has second thoughts about mentioning Zahar, whose life might now be in danger. If Ambrimos was in cahoots with Abu bin Hamud, he would certainly not want Zahar around as a witness. Biggles spots a car leaving the villa. Believing this must be going to fetch Zahar, he waits. A short time later, the car indeed returns with Zahar. While stopped at the gate, Biggles opens the car door and drags Zahar out, holding the protesting servants of Ambrimos at bay with his pistol.
At first Zahar doesn't understand--he thought Ambrimos merely wanted some information and had promised to pay for it. But Biggles explains his suspicion about Ambrimos being linked to Abu bin Hamud, and this convinces Zahar. He promises to enter into service with Biggles--it would also be the best means for him to settle his score with Abu.
Zahar tells Biggles he, like Abu, used to smuggle hashish for Ambrimos from El Moab, a farm in the Danakil Desert where the hemp crop was grown. This worries Biggles--now Ambrimos might have the gurra seeds, he has a place to grow it, and probably, with his hashish channels, a means to distribute it as well. As an aside, Zahar mentions that an aircraft was sometimes used to smuggle the hashish.
Chapter 6: The Enemy Hits Back[]
The next morning, Biggles, Ginger and Zahar take off in a Proctor for El Moab but someone has sabotaged the aircraft by cutting a slit in the wing, causing the fabric to balloon. They are forced to return to Aden and leave Bertie to look after the repairs. Meanwhile they take off in the other aircraft and after spotting El Moab from the air, head to Aerodrome 137, a disused R.A.F. landing ground nearby--Biggles had earlier told Bertie to rendezvous there.
Chapter 7: Nature Intervenes[]
Down at Aerodrome 137, Biggles, Ginger and Zahar are caught in a severe sandstorm.
Chapter 8: Ginger Falls Out[]
The storm passes as night falls. Knowing Bertie would not try to fly out to them until the weather conditions improve, Biggles decides to do a ground reconnaissance of El Moab. Along the way, Ginger takes a drink from a stream and starts to feel ill. Biggles tells him to wait there while he and Zahar complete the mission. But Ginger, in a stupor, moves off into the desert and becomes lost.
Chapter 9: Zahar Solves a Mystery[]
Biggles and Zahar press on to El Moab where Biggles confirms that a crop of gurra is being grown together with crops of hemp. He also sees a De Havilland Gipsy Moth which is obviously used to transport the finished products. They return to pick up Ginger but he is nowhere to be found! Zahar now discovers that the water Ginger drank had hashish in it--it flowed through El Moab and had been used to treat the crops after harvest. After a long and fruitless search for Ginger, they return to Aerodrome 137.
Chapter 10: Bertie Gets a Fright[]
Bertie flies to Aerodrome 137 in the morning and waits for Biggles. Not expecting anyone else, he is surprised to see a Gipsy Moth landing. The pilot begins to refuel the aircraft from an underground tank. Bertie does not know the Moth comes from El Moab so he goes up to greet the pilot who returns the courtesy by shooting at him. Bertie is taken aback and returns fire. The pilot attempts to take off in the Moth but Bertie shoots out a tyre whereupon the pilot flees on foot.
Biggles and Zahar return. Biggles is dismayed to find that the hangar where he had parked his Proctor has collapsed under the weight of sediment deposited during the sandstorm. A brief examination shows that the aircraft appears all right but some girders need to be cut away.
Chapter 11: Biggles Takes a Ride[]
Biggles sends Bertie back to Aden to fetch an acetylene torch and someone who can use it. He is also to bring some dynamite as Biggles has decided to blow up the dam at El Moab as the best way of getting rid of the gurra. Finally, Bertie is also to bring a spare wheel for the Moth.
A large group of horsemen are seen approaching so Biggles tells Zahar to remain hidden in the hangar clearing the Proctor and tell Bertie what happened. The horsemen want Biggles to go with them because they have Ginger. Biggles goes along but it turns out they are working for Ambrimos. Biggles is brought to El Moab and taken prisoner--Ginger is already there.
Chapter 12: The Sultan Shows His Hand[]
Ambrimos, who must have flown into El Moab by the Gipsy Moth, makes an offer to Biggles. He will pay Biggles well to drop the investigations and even work for him as a pilot. Biggles refuses and Ambrimos launches into a tirade against the Britain, threatening to bring it down with gurra. He then tells Biggles and Ginger that he has no choice but to put them to death but first he will give them some time to change their minds.
Chapter 13: Zahar Settles a Debt[]
Bertie returns to Aerodrome 137 with L.A.C. Blakey, an acetylene welder. Together they change the tyre of the Gipsy Moth. Zahar tells Bertie what has happened to Biggles.
Bertie leaves Blakey to cut away the collapsed hangar girders and free the Proctor. He flies the Moth with Zahar to El Moab to blow up the dam. Bertie has almost completed planting the explosive when he is shot at. Abu bin Hamud, probably out on patrol, has seen him. Bertie is outgunned and has little cover. Moreover, he had dropped the match and had lit the fuse! Fortunately Zahar has been mounting overwatch and he attacks Abu, killing him and settling his score with him.
Bertie hears a commotion in the direction of the farm and proceeds to find out what it is about.
Chapter 14: Ambrimos Gets His Answer[]
Ambrimos wants a final answer from Biggles and it is still no, so Biggles and Ginger are brought out to be hanged at a tree. Bertie spots this gathering. Although totally outnumber, he begins shooting at the assembly. Crazy, thinks Biggles, but just then, there is a massive explosion followed by a flood of water. Biggles and Ginger escape by climbing up the rope which would have been used to hang them, but most of the other people around them are swept away and probably drowned. Our friends then taxi back to Aerodrome 137 in the Moth.
Chapter 15: Final Details[]
Everyone flies back to Aden. Zahar is rewarded for his part in the venture. Back in London, Biggles later receives a letter from Captain Norman telling him that nothing further had been seen of Ambrimos and that he is believed to have died at El Moab.