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Biggles investigates an unidentified aircraft that is persistently flying into the country under the radar curtain. The plane is always heading towards the Cornwall/Devon region, so Biggles concentrates enquiries there. Alerted by the murder of a police constable at Bodmin Moor in Cornwall, Biggles, Algy, Ginger and Bertie go there to make further enquiries. Aerial photographs of the Moor reveal a burnt strip of land and a fresh green strip parallel to it. This is next to an abandoned mine where there is located a mysterious shepherd with a fierce dog. When Ginger investigates the mine, he finds himself held at bay by the dog. Things start to move when 'Cracker' Lewis escapes from nearby Dartmoor prison. Investigating a suspicious Bentley car, Bertie finds out that Sir Humphrey Trethallan who lives at nearby Hallstone Towers owns it. Keeping watch on Hallstone Towers Biggles is shocked to see Sir Humphrey with Lewis. Biggles suspects the intruding aircraft is flying escaped criminals out of the country in return for part of their stolen loot and Sir Humphrey is behind the scheme. Bertie is assigned to observe Hallstone Towers and he sees Sir Humphrey leading Lewis out to Bodmin moor. Bertie then accidentally falls down an abandoned mine shaft and is knocked unconscious. Coming to, he is confronted by the shepherd and realises that the shaft he has fallen down must connect to the abandoned mine under suspicion. With Ginger's help, Bertie gets out of the mine. The intruding aeroplane arrives and Biggles, with the help of the local police move in to make arrests. Sir Humphrey escapes and is pursued back to Hallstone Towers. Here, he blows his own brains out with a gun, rather than be arrested. The cover of both the first edition paperback and later hardback show Biggles (presumably) with the Bentley car and the 'dark intruder' aircraft in the background.