pp. 255, illusts (no dustjacket) #041121 (Originally published 1940)

From biggles.info: Professor Max Beklinder has disappeared. He is the inventor of Linderite explosive, a Lucranian by birth but British by naturalization. The Professor was working on a secret poison gas for the British when he was reported as having been killed in a car accident in Unterhamstadt in Lucrania. Biggles is sent to find out if he is really dead and if not, to get him back. Algy flies over to Lucrania and Biggles and Ginger parachute down. They intend to pose as British tourists and try to establish the truth about the Professor’s death. It’s not long before they discover the Professor’s Morris Ten car completely undamaged, hidden in a nearby castle. Staying at a hotel in Unterhamstadt, Biggles and Ginger set off to explore the vault where the Professor is supposedly buried. In his coffin, they find only books, but a mysterious girl has followed them. The girl is staying at Biggles and Ginger’s hotel. The local police raid the hotel and capture the “girl” and it turns out that it is really a man in disguise – Beklinder’s son. Lucrania is under the heel of one of its neighbours, and a native of that country is in charge of the police. Erich von Stalhein is at the hotel checking the names of the guests and is surprised to encounter two of his old foes. With minutes to spare, Biggles and Ginger escape into a secret passage that leads from room 17 in the hotel to the castle. The passage leads behind the castle walls and Ginger comes across a spy hole into a room that is being used as von Stalhein’s office. To Ginger’s astonishment he sees that Algy is a prisoner! Von Stalhein also has Professor Beklinder as a prisoner. The Professor threatens von Stalhein with some powerful explosive he has made but von Stalhein has a trump card to play in the form of Beklinder’s captured son. Beklinder has to hand the explosive over. Von Stalhein plans to take the Beklinders away and Biggles and Ginger know they must stop this. They capture the car taking the Beklinders out of the castle and race to find Algy’s plane but it is too late as von Stalhein has had the plane removed. Biggles has a plan and tells Ginger that he and the Beklinders must meet him at a certain rendezvous at 12.20 a.m. In the meantime they are to hide up at the vault while Biggles tries to organise some transport. In the vault, the Professor tells Ginger that there is also a secret passage from the vault to the castle. Using this, Ginger returns to rescue Algy, something he succeeds in doing with a bit of help from Beklinder’s explosive. Ginger and Algy rush to the vault and find that the Beklinders have already set off. Catching up with them, the four men wait at the rendezvous, but already the soldiers are out tracking them with dogs. Meanwhile Biggles has gone to Prenzel, as he knows that a regular mail plane leaves for Croydon at midnight. He hopes to persuade the pilot to help him. Things don’t quite go to plan and Biggles has to steal the plane. He flies to the rendezvous and is just in time to load his passengers aboard and make a desperate flight back to England.