In Hong Kong, International Air Police agents mount surveillance on one Rex Barker, a suspected of being a drup smuggler specialising in bringing large quantities of heroin to Europe. He seems to be making a visit every two days to meet a certain Nancy Towang at a small Chinese restaurant.
The surveillance tape are delivered to Colonel Lottison who puts Biggles on the case. According to Lottison, Barker's gang is suspected of having established an operating base on the island of Kwan-Tia in the South China Sea. From the next day, reconnaissance aircraft would begin patroling the area to hunt for Barker's ships. Meanwhile Biggles must find out what Barker is doing at the restaurant.
Biggles stakes out the restaurant and spots Baker going into the restaurant and then into a private room upstairs where he meets Nancy. Biggles plants himself on the balcony outside and hears Barker forcing Nancyy to contact her brother Chin Lee on the radio. Chin Lee tells her that reconnaissance flights are about to be launched from Kwan-Tia. Barker is annoyed to learn this. He says now is the time for Chin Lee to prove his worth. His sabotage efforts must succeed with the very first reconnaissance flight.
Nancy spots Biggles in the balcony and whispers a warning to Barker. Barker sneaks out and attempts to capture him. There is a scuffle which ends with Biggles being knocked out. Barker leaves to get some men to dispose of Biggles. Left alone, Nancy doesn't want to see any killing so she drags Biggles and hauls him over the balcony rails where his fall is broken by the canvas awnings and baskets of the market stalls below. Barker sends his men after Biggles but the police, attracted by the commotion, get there first.
BIggles hurriedly notifies Kwan-Tia about the intended sabotage but too late--Ginger has already taken off in his F-8 Crusader. Ginger is recalled but his engine flames out and he is forced to eject. Ginger can't explain what happened to the aircraft. The wreckage is recovered from the sea but they can find anything wrong with it either. Flights are suspended while Biggles and Ginger inspect the island in a jeep.
They drive out to a fishing village at the end of the runway. The fisherman and his wife are clearly terrified and the reason is soon obvious: a thug takes a shot at Biggles. Biggles and Ginger pursue the thug but he trips and falls over a cliff to his death. He was probably one of the saboteurs and had come to the island a few days before. But he is dead, so Biggles still doesn't know how the sabotage took place.
Flights resume, but the next aircraft, a Lockheed T-33 piloted by Jesse Bowers, also flames out. Biggles takes off to look for him and also flames out but he manages to glide the aircraft onto some rocks before ejecting. Biggles is injured and taken to hospital in Hong Kong. Colonel Lottison tells him some startling news: the fuel in his aircraft has been analysed and found to be totally decomposed. Meanwhile, the police had identified the thug whom Biggles and Ginger chased. He has a brother in Hong Kong and may be a lead but he is murdered before the police can get to question him.
A few days later, there is a fresh break: Barker's car has been spotted near a Chinese temple. Biggles goes there and spots Nancy Towang getting into it. He follows her to the harbour where she boards a sailing junk. Biggles sneaks aboard where he spies on Nancy talking to Barker and demanding to see Chin Lee, who is brought up on deck. It turns out Chin Lee has been Barker's prisoner--some of his scientific work has been useful to him. Meanwhile the two must remain on board until Barker's convoy of junks has delivered its cargo of narcotic drugs.
Biggles goes below to search the cabins, starting with that of Chin Lee. It is a fully equipped workshop with power tools and chemicals. But how he performs the sabotage is still a puzzle. In a chest, Biggles discovers a book about ... birds?
Barker tells a crewman to lock Nancy and Chin Lee up in the hold. Once below deck, the crewman is preoccupied with a stuck key and momentarily turns his back on the prisoners. Nancy seizes the moment to grab his gun and Chin Lee joins the scuffle. But the crewman regains his gun and soon Barker joins him. He knows a way to keep them out of mischief--by injecting them with a sleeping drug. After that, he intends to send a signal to tell his convoy to sail.
Biggles overhears this, and goes in search of a radio in order to alert the base at Kwan-Tia. Not finding it down below, he goes on deck but has to fight off a number of crewmen who have spotted him. There is an exchange of gunfire, and the crew rolls out a hidden machine gun, forcing Biggles to take cover. Just this moment, Barker comes up on deck to see what is going on. He is in the line of fire, Biggles is behind him and seizes him as hostage. The crewman are forced back and down into the hold but one of them, unseen, swings the jibe of the sail and knocks Biggles out.
Biggles awakes in the hold, all tied up. Nancy and Chin Lee are next to him, still unconscious. He breaks free of his bonds and searches for a way out and has a stroke of luck when he finds a box of life-vests. He puts them onto Nancy and Chin Lee and lowers them into the water through a porthole, before joing them himself.
On board the junk, the escape of the hostages is discovered but Barker does not have time to turn back to look for them. He radios his fleet and orders it to sail. One of the junks has an unserviceable motor and leaks but he orders his men to man the pumps and take the vessel under tow: the convy must move!
Meanwhile, Biggles, Nancy and Chin Lee are rescued and taken to a hospital in Hong Kong. With the two still unconscious, Biggles is unable to question them about the method of sabotage and so Lottison cannot order the flights to resume. Biggles decides to fly to Kwan-Tia. Approaching the threshold, some ducks fly into his path. Chin Lee had a book about birds. Was this a clue?
Upon landing, Biggles calls to Ginger to lay on a jeep and a camouflage net. They are going to catch some ducks! Biggles nets some ducks but can find nothing unusual about them. Then Ginger notices something unusual. All the nests seem to be on a straight line. He searches the nests and finds gadgets fitted with strong magnets. Whatever they are, that's how the aircraft are being sabotaged.
With the gadgets gathered up, the fighters at Kwan-Tia are free to scramble and Biggles leads a flight which intercepts the drug convoy and sinks it.
Back at Hong Kong, Chin Lee fills in the gaps. Barker had threatened his sister Nancy's life, forcing him to cooperate. The gadgets were planted in nests at the end of the runway where, because of their strong magnets, they would attach themselves to Kwan-Tia's aircraft as they took off. Then some kind of beam emitted by the gadget would decompose the fuel on board. Biggles tells them Chin Lee's invention would be handed over to the authorities and who knows where one might end up from a study of ducks.