Biggles, Algy and Ginger undertake a 'suicidal' task for the British Government. They are sent to establish whether the death of a naturalised British subject, Professor Beklinder, in Lucrania is fact or fiction.

goodreads summary

This is the story of how Biggles missed a visit to the pictures and chanced upon one of his most hazardous adventures.

The facts were laid before him. Professor Becklinder, a naturalised Lucranian, had discovered the formula for a deadly poisonous gas. While on sick leave from England he had been reported killed, but in doubtful circumstances.

British Intelligence were vitally interested in establishing the truth behind the report, and it was Biggles' job to discover whether the Professor was really dead and, if not, who had captured him and where he was.

The first grisly step in the investigations was to open the Professor's grave an examine the body, if one existed - a horrible undertaking, but one that desperate necessity justified, in Biggles' view.

Intrepid as ever, he and his friends, Algy and Ginger, ploughed through the whole dangerous gamut of espionage, captivity, escape and rescue, in their attempts to solve the mystery of the Professor's disappearance.