I am not able to understand the story of Evans tries an o-level. Though i've read the chapter summary and the entire chapter many times. Please make it clear in simple words the escape plan of evans...

James Roderick Evans was a prisoner who had made a reputation for himself as 'Evans the Break' on account of escaping thrice from the prison. As he started his night classes in O-Level German, the government arranged to allow him to sit for his O-Level examinations in the prison cell. On the day of the exam he was given a smart look but he refused to remove the cap as he considered it to be a lucky charm. The final plan of escape unfolded when after seeing McLeery off, the officer took one last look at Evans and saw it was the former sprawled on Evan's chair. He was in bad shape but instead of calling the ambulance, he asked for the police because he knew where Evans was. He opened the German question paper and found a photocopied sheet carefully and cleverly superimposed over the last page of the question paper. The instructions were written in German asking him to make his way to Neugraben. The governor wanted to know about the false beard, spectacles and other things but he was puzzled that McLeery was not in hospital.An ambulance was sent to Elsfield but the person in question had vanished. They found him bound and gagged after some time and it was then that they understood that Evans had been impersonating as McLeery who had walked out. It turned out that Evans had walked out and McLeery stayed.