During the Second World War it was difficult to travel by plane, because the seats were needed for important government and army people.
Mr Brown worked for the government during the war. He was a civilian, and he was doing very secret work, so nobody was allowed to know how important he was except a very few people.
One day he had to fly to Edinburgh to give a lecture to a few top people there, but an important army officer came to the airport at the last minute, and Mr Brown's seat was given to him, so he was not able to fly to the city to give his lecture.
It was not until he reached the city that the important officer discovered that the man whose seat he had taken was the one whose lecture he had flown to the city to hear. 
Government/noun/: ruling of a country or area دولت، حكومت، فرمانداري
Allowed/past tense of allow/: permitted, authorized اجازه دادن،
Civilian ordinary citizenشخص غير نظامي،
Discovered/past tense of the discover/: find, gain knowledge, learn of - پي بردن، دريافتن، يافتن
Reached/past tense of the reach/: arrive; obtain, procure رسيدن به،
