in ▁characters ▁were ▁developed , ▁and ▁while ▁pre - production ▁had ▁begun , ▁the ▁film ▁project ▁was ▁eventually ▁abandoned , ▁in ▁part ▁because ▁the ▁studio ▁could ▁not ▁establish ▁the ▁precise ▁rights ▁of ▁the ▁" gre ml in " ▁story , ▁and ▁in ▁part ▁because ▁the ▁British ▁Air ▁Ministry ▁was ▁heavily ▁involved ▁in ▁the ▁production ▁because ▁D ahl , ▁who ▁was ▁on ▁leave ▁from ▁his ▁w art ime ▁Washington ▁posting , ▁ins isted ▁on ▁final ▁appro val ▁of ▁script ▁and ▁production . ▁ ▁Plot ▁The ▁story ▁concerns ▁m isch iev ous ▁myth ical ▁cre atures , ▁the ▁Gre ml ins ▁of ▁the ▁title , ▁often ▁invoked ▁by ▁Royal ▁Air ▁Force ▁pil ots ▁as ▁an ▁explanation ▁of ▁mechanical ▁troubles ▁and ▁m ish aps . ▁In ▁D ahl ' s ▁book , ▁the ▁gre ml ins ' ▁motiv ation ▁for ▁sab ot aging ▁British ▁aircraft ▁is ▁reven ge ▁of ▁the ▁destruction ▁of ▁their ▁forest ▁home , ▁which ▁was ▁raz ed ▁to ▁make ▁way ▁for ▁an ▁aircraft ▁factory . ▁The ▁principal ▁character ▁in ▁the ▁book , ▁G us , ▁has ▁his ▁Haw ker ▁H urr icane ▁f ighter ▁destroyed ▁over ▁the ▁English ▁Channel ▁by ▁a ▁gre ml in , ▁but ▁is ▁able ▁to ▁conv ince ▁the ▁gre ml ins ▁as ▁they ▁par ach ute ▁into ▁the ▁water ▁that ▁they ▁should ▁join ▁forces ▁against ▁a ▁common ▁enemy , ▁Hitler ▁and ▁the ▁Naz is , ▁rather ▁than ▁fight ▁each ▁other . ▁ ▁Event ually , ▁the ▁gre ml ins ▁are ▁re - tra ined ▁by ▁the ▁Royal ▁Air ▁Force