In Biggles in the Orient Lal Din was a mess steward at Dum Dum airfield and also an enemy agent whose job it was to plant doped chewing gum or chocolate bars into the cockpits of aircraft, thus causing the pilots who eat them to lose consciousness and crash.
Lal Din was described a moon-faced man, short, stoutish, olive-skinned, and middle-aged. He had been employed by the canteen manager Ali Mansur at the recommendation of Larapindi. When Biggles asked where he came from, Lal Din told him that he was Burmese, from Mandalay. His later behaviour showed that he had Japanese ancestry.
The strategy of planting doped candy in aircraft had one limitation--the candy had to be carefully controlled. Only aircraft detailed for sorties should have them. Unused candy had to be retrieved. Otherwise people might get hold of them and start getting doped while on the ground. After the effects wore off, the victims would remember what happened and thus give the game away. By listening to conversations in the mess and reading the daily orders on the bulletin board, Lal Din was able to learn which aircraft had been detailed for flights for the day and so act accordingly. It was believed that he made use of innocent parties like the fitter Sergeant Gray to plant the candy in the cockpits as a mess steward would not normally have access to aircraft.
However things went wrong for Lal Din when Sergeant Gray, upset by all the deaths in his squadron, got hold of some chewing gum from the cockpit of an aircraft and started to consume it. Not long after that, he was discovered unconscious on the hangar floor by Biggles, Johnny Crisp and Air Commodore Raymond. They thought Gray was drunk, but by forcing some coffee into him, Biggles managed to make Gray recover enough to mutter a few words about what really happened--he denied being drunk, he had only been "chewing things over". Lal Din had heard this as he had fetched the coffee and knew that he was on the brink of being exposed. Later, when Gray was alone and resting in his room, Lal Din used the pretext of bringing Gray another pot of coffee to enter his room and suffocate him.
But Biggles had already surmised what had happened and shared his suspicions about Lal Din with the Air Commodore. Raymond was concerned to eliminate the threat as soon as possible and urged Biggles to question Lal Din straightaway. Biggles, yielding to the Air Commodore's pressure, confronted Lal Din in the mess and tried to force him to eat a bar of chocolate in front of him. Lal Din, knowing he had been cornered, broke free from the restraints of the 666 Sqn officers and hastily committed ritual suicide with a knife--an act which suggested to Biggles that he had Japanese ancestry.