Biggles and his 666 Sqn are taken to Malta, then Alexandria, Baghdad and Karachi, not being told where their final destination is. They finally arrive at Dum Dum, Calcutta where, to his surprise (or perhaps he ought not have been), Biggles is met by Air Commodore Raymond.
Biggles is taken to a briefing with the A.O.C. and G.O.C India where he is told that aircraft and pilots have been disappearing mysteriously while flying the India-China air route which ferries vital war supplies to allied Chinese troops. The situation has become so bad that the route has virtually shut down. Worse, the phenomenon has spread to other air stations across India, including Dum Dum.
The strangest aspect of these events is the seeming lack of pattern. Aircraft disappear at various points along their routes but others get through without any sign of interference. At times, parts of a formation are affected but others are not, and some pilots always seem to escape harm. The group discusses various possibilities such as an enemy death-ray but Biggles dismisses them because none of them fit the pattern of disappearance, or the lack of pattern.
Biggles begins by flying alone to Jangpur, the southern terminal of the India-China air route and where the disappearances began. He finds Squadron Leader Frayle, the station commander, in a state of near-nervous breakdown, having lost numerous pilots and aircraft in the past few days. After some encouraging words for Frayle, Biggles decides to take a Wellington on a run to China. Bargent, the pilot who originally volunteered for the flight insists on coming along and Biggles agrees. They take precautions, changing aircraft at the last minute and loading the cargo themselves. They make it to Chungking, the China terminal and return uneventfully, Biggles winning a wager of 100 cigarettes he made with Bargent that they would make it.
The next day, Biggles shadows a Hurricane fighter bomber raid led by Johnny Crisp. Following in a Mosquito, Biggles sees nothing eventful during the raid but on the return trip, he sees one of the aircraft slowly dropping out of formation and then crashing into the jungle. He parachutes down to investigate, telling Tug Carrington, his co-pilot, to come back and pick him up at a nearby river in a marine aircraft.
Down in the jungle, Biggles finds the wrecked Hurricane and examines it thoroughly but finds nothing wrong. Of the dead pilot, Thomas Grafton Moorven, Biggles notices only his dilated pupils. Biggles also finds a chewing gum wrapper but doesn't attach any importance to it.
Interrupted by two Japanese soldiers, Biggles hides. However, he is soon angered to see the soldiers treating Moorven's body with disrespect and shoots them both. He then burns the aircraft, with Moorven in it, throws him a salute and leaves for the river to await Tug.
Tug arrives in a Moth seaplane and Biggles is surprised to see a Hurricane piloted by Angus Mackail flying escort. Suddenly, Angus appears to lose control and crashes into the river. Biggles dives in and pulls Angus out, putting him into the Moth and telling Tug to fly Angus to a hospital and then come back for him.
Back at Calcutta, Biggles learns that three of Johnny Crisp's formation of five aircraft, including Moorven, had failed to return. Biggles talks to the survivors, Crisp and Scrimshaw but learns that they had done nothing different from the other three. They decide to speak to Sergeant Gray, the fitter who last handled the aircraft. They find him in a hangar, apparently drunk. Biggles sends Lal Din, the mess steward, for some coffee. Gray begins to come around and insists that he had not been drinking. Gray is unable to talk any further so they leave him.
Hours later, Crisp calls Biggles to tell him that Gray is dead! Biggles rushes over to examine the scene--there was no sign of injury on Gray. Suicide? Heat-stroke due to excessive drinking? Biggles is not so sure. He notices a scrap of chewing gum wrapper and a coffee pot which is still warm.
Next day Biggles tries an experiment. He flies his Typhoon, leading Ginger and Johnny in Hurricanes ostensibly for a patrol but instead they land at the practice landing strip at Gayhar where they sit in the aircraft with engines running. Nothing happens for a while but suddenly, Johnny slumps over his controls. Biggles hurriedly drags him to a waiting ambulance--he had earlier arranged this with Algy, and administers a medicine to make Johnny throw up. Johnny comes round and Biggles begins to question him urgently: did he take any of the chewing gum in his aircraft? Johnny insists he did not--he hates gum. Biggles is puzzled but he takes a look around the aircraft and then sends them all back to Calcutta.
Back at the station, Raymond has urgent news for Biggles: a large blitz of 98 Mitsubishi bombers is headed for Calcutta! All the station can spare is six Spitfires to intercept. Biggles disagrees. He convinces Raymond to launch every available aircraft but first recall the Spitfires and parade the other fighters and their pilots. Biggles hastily makes the rounds of the aircraft, says a few words of encouragement and then orders everyone aloft.
The ensuing air battle is a great success--they shoot down more than 30 bombers and no one is seriously hurt--except for Scrimshaw who, upset about the deaths of his squadron mates, deliberately rams his Hurricane into a enemy bomber. Strangely, no friendly aircraft succumb to the mysterious phenomenon of dropping uncontrollably out of the sky.
Gathering all the pilots and Raymond in the mess, Biggles explains what happened and produces a chocolate bar. He suspected the confectionery in the aircraft had been laced with a narcotic and had quietly removed them all from each cockpit before they took off. He explains his deductions. At first he suspected chewing gum because he had found gum wrappers at Moorven's wreck and then in Gray's room. His theory had been temporarily derailed when Johnny collapsed but had not taken any gum but later Biggles had found a chocolate wrapper in his cockpit. Biggles suspects that the various stations must have an enemy agent inside who had been planting then spiked confectionery in the aircraft cockpits. In their case, he suspects Lal Din because he is well-known enough to move around freely. He was also one of the few people who knew about Sergeant Grays "drunkenness" and being the enemy agent, had to silence Gray before he came around and told the others that he had been chewing gum and not drinking. Biggles had also found the a warm coffee pot in Gray's room which suggests Lal Din might have returned to Gray, using a fresh pot of coffee as an excuse.
Raymond is jubilant and wants to confront Lal Din immediately in the hopes of extracting information about the chief agent behind the whole scheme. Biggles reluctantly agrees and asks Lal Din to come in and tries to force him to eat a chocolate bar. Lal Din, knowing he is trapped, pulls out a knife and commits suicide before anyone can stop him.
Biggles decides to learn more about Lal Din's movements to get fresh leads. Together with Algy, he questions Ali Mansur, the mess manager, who tells them Lal Din had been recommended to the mess by Mr Larapindi, a rich merchant from the import-export firm Tahil and Larapindi. It was this firm which had supplied boxes of chewing gum and other confectionery for free. Lal Din had a brother who called for Lal frequently but not that day.
Satisfied, Biggles takes things a step further by going to Jangpur. From there he takes off in a Wellington, intending to make a night trip to Chungking but returns a few minutes later, emerging from the aircraft looking disorientated and apparently drunk. Frayle (who had been forewarned by Biggles) and Bargent help Biggles to a room and settle him on a bed.
About nine-thirty an intruder creeps into Biggles' room and attempts to strangle him--but Biggles has been expecting him--he had a trap! Biggles tries to restrain the intruder but found that he wasn't wearing clothes and had oiled his entire body. The intruder almost escapes so Biggles has to shoot him. Frayle and Bargent rush over. It turns out the intruder is Kong Po, the laundryman. Biggles tells Frayle Kong Po is an enemy agent and gives him the background. He tells Frayle that supply flights to Chungking can resume the next day but pilots were to be told confidentially to avoid eating anything during the flight.
With two agents out of the way, Biggles has to move fast before the head of the network becomes alerted. Back at Calcutta, he learns that Lal Din's brother had come asking for the mess steward. Algy had followed him as he left, and had ended up at the warehouse of Tahil and Larapindi. Biggles prepares his squadron for a raid on the warehouse but first he pays a call on Mr Larapindi.
At Larapindi's, Biggles pretends to be the mess president and tells Larapindi that Lal Din has come down with small pox. Could he recommend someoone else? Larapindi assures him a new man will be sent the very next day. Biggles also notices a picture of a Gull seaplane on the wall--Larapindi explains that it is a company aircraft used for visiting their various branches all over India.
Taking his leave, Biggles and the others wait outside and soon enough, they observe Larapindi leaving in a car and follow him to the warehouse. Planting his squadron in a cordon around the building, he takes Algy and Ginger stealthily inside. Biggles finds Larapindi but is surprised that he is in a room with more than eight others, more enemy agents it seems and apparently involved in some religious ritual in front of a green idol. In the ensuing shoot-out, Larapindi makes his escape.
Outside the warehouse, Biggles learns from Bertie that Larapindi was last seen escaping upriver in a boat. Biggles now remembers the picture on the wall and deduces that he must be heading for the Gull. He sends Bertie and some others in a lorry to follow up the river while he takes Larapindi's car back to the station to pick up a Spitfire.
Once in the air, Biggles searches the river for Larapindi or the Gull. He becomes distracted by flashing lights from the ground (Bertie was signalling his position to him) and almost collided with the Gull. Turning around, he pursues Larapindi hoping to force it to land, but when the Gull opens fire at him from a mobile machine-gun, Biggles shoots it down.
Back at the warehouse, things were being wrapped up. Raymond had arrived and had taken charge. Besides the laboratory full of incriminating narcotics, several enemy agents had been captured, as well as some Japanese airmen, downed aircrew from the battle in the morning who had been using the warehouse as a safehouse. In Larapindi's safe, they had also found details of the enemy agents in the other stations. by dawn, Raymond predicts, the entire enemy spy organisation would be wiped out.
Satisfied, Biggles takes his leave. "Let's go, before the Air Commodore thinks of another tangle for us to straighten out," he tells Algy.