Biggles and his cohorts in 266 Squadron are sent to Calcutta to uncover a sinister secret weapon that is decimating flights from India to China.

After watching in horror a few colleagures crash for no reason, including Angus from 266 itself, Biggles manages to uncover that the secret weapon is doped sweets left in the aircraft cockpits by a fifth columnist. Initially chewing gum, the trick turned to chocolate later as suspiciously the same pilots kept escaping the fate of others; turns out they were the ones who did not like gum.

Biggles uncovers the spies in the midst of each aerodrome, and they die before spilling the beans over their supplier. However some further investigation tracks all supplies to a single distributor in India. With thanks to 266, they too meet a "sweet" end.