Biggles is attending an Interpol conference Paris with his French counterpart Marcel Brissac and also a young American policeman named Eddie Ross. Ross has just given a strident speech at the conference hoping to convince the international police community to focus on international drug trafficking . The problem has become so bad in the United States that youths are being handed confectionery spiked with heroin.
The three policemen stop for refreshment at a Parisian street cafe where Biggles advises his American friend to keep a low profile because the drug gangs can be ruthless. They might already have targeted Ross because of his speech. Sure enough, Biggles spots a man who has been shadowing Ross seated a few tables away. Believing this might be a member of a drug gang and sensing an opportunity to turn the tables, Biggles works out a plan. First Marcel takes his leave and accidentally knocks into the man to get a good look at him. In the meantime, Biggles and Ross take a leisurely stroll to Biggles' hotel, knowing that they will be followed.
At the Hotel Pont-Royal, Biggles gets a call from Marcel who tells him he has identified the man as a gangster named Georges Macula, who has a friend Del Grikko who runs a bar named "The Laughing Horse". From his window, Biggles sees Macula outside watching their hotel. A cab draws up and a man steps out to speak to Macula. Biggles sends Ginger off in a cab to shadow the man as he leaves.
Ginger's cab follows the man to the "Laughing Horse" bar. There, Ginger takes a table and orders some coffee. He is approached by an Englishman, Noel Burton, who is desperate for some money to buy drugs but Ginger declines to help. Before Ginger can do anything more, he collapses. His coffee, it appears, has been drugged. He is brought into Del Grikko's office who demands why Ginger had been following his man to the bar.
But Biggles has not been idle. He had noted Ginger's cab number, and with Marcel's help, he traces the destination of Ginger's cab. Biggles and Ross go to "The Laughing Horse' where they meet Noel. Noel is induced with some money to talk. He tells Biggles that drugs are distributed at the bar and points out a gaudily dressed Egyptian named Alfondez, the runner who delivers the drugs. Biggles sends Ross to shadow Alfondez as he leaves. Burton also tells Biggles that Ginger was carried into Del Grikko's office. Biggles storms into the office to confront Del Grikko, wanting to take Ginger away. The atmosphere is tense and Biggles is outnumbered, especially when Macula also turns up but fortunately, Marcel arrives and they are able to leave safely.
The next morning, Ross calls Biggles from Marseilles! He had followed Alfondez all the way there and had seen him board a ship, the Saphos. Ginger and Marcel proceed to meet Ross there while Biggles first flies to London to bring fetch his Otter amphibian together with Bertie, before flying to Marseilles to collect the others.
The Otter pursues the Saphos across the Mediterranean towards the Greek islands in the Aegean Sea. Bertie now has a bright idea" he wants to be dropped out at sea in the path of the Saphos where he will pretend to have fallen overboard. He hopes to be picked up by the ship and so will be able to learn its destination. Moreover, his "accident" will give an excuse for Biggles' aircraft to be seen flying around ostensibly looking for him.
Biggles reluctantly agrees to the idea and Bertie is cast off in a dinghy. He is soon picked up by the Saphos whose crew includes Alfondez. In conversation, Bertie hints that as a Lord, he doesn't need to work except for excitement. Alfondez takes the bait and asks if Bertie isinterested in easy money. Bertie plays along and soon, after the ship docks at a Greek island, Bertie is taken to meet Ali, Alfondez's brother, who offers Bertie an opportunity for both money and excitement by using his position to provide an easy channel for running drugs into Britain. Bertie sounds interested, and they take him blindfolded on a mule to an abandoned monastery in the mountains on the island to meet Colonel Nicolinos, the leader of the drug gang.
Unfortunately Colonel Nicolinos has been in touch with his agents in London by radio and has learnt that Bertie is a member of Scotland Yard. He interrogates Bertie wanting to know how he has stumbled upon his island hideout so quickly. In a tight spot, Bertie tells Nicolinos he has a man named Alfondez--"Ask him."
Meanwhile, Biggles and his friends have been shadowing the Saphos. They lost it for a while but soon discover it docked at the Greek island of Venesos, next to a much larger ship. They put the Otter down in a hidden creek and set off on foot for a reconnaissance. Along the way, they meet Alfondez who is injured and on the run. The Colonel's men had been after him ever since Bertie had incriminated him. He promises to help Biggles if he would protect him from the gang and takes Biggles' party to the monastery.
Approaching the monastery, they hear Bertie singing loudly, giving the location of the prison cell he has been locked up. Alfondez sneaks away but he doesn't get far. He meets a group of the Colonel's men who pursue him and shoot him fatally. Biggles and Ross take advantage of the commotion to steal into the monastery where they release Bertie. They also find the chapel--which has now become a lab for the production of heroin from opium brought in by the ships. After setting fire to the lab, they are about to make their exit but then the Colonel and his men run in through the door. They have discovered Bertie's cell is empty and are in pursuit. There is a shootout but the fumes from the burning chemicals in the lab provide enough cover for Biggles and his friends to make their escape. On the way out, Biggles sees Ali stab the Colonel in the back with a knife--in revenge for killing his brother.
Biggles and his friends now head to the aircraft but finding their way blocked, head for the village where they take the opportunity to set fire to the two ships. They then take a boat and row towards the creek where the Otter is moored.
Departing the island, the Otter is short of fuel but fortunately Marcel spots a British aircraft carrier, the H.M.S. Ark Royal, which comes to their aid. Returning to London, Biggles tells his boss, Air Commodore Raymond, that he and his friends have just destroyed a drug gang in a combined operation but he should not ask too many questions so that he can plead ignorance should there be international repercussions. Raymond remarks that one of these days, Biggles will get him in trouble. Biggles responds that it is more likely he will get himself into trouble.