Theory:

INTRUDER: (rattles door and shouts) Let me out of here!
 
GERRARD: Hello. Yes, speaking. Sorry I can’t let you have the props in time for rehearsal, I’ve had a spot of bother — quite amusing. I think I’ll put it in my next play. Listen, can you tell our friend the Sergeant to come up here at once? You’ll probably find him in the Public Bar.
Explanation:
 
The intruder's game was finally over, as he was trapped within the cupboard. He kept hammering on the door, asking to be let out. Gerrard's intellectual capacity shone through not just in his words but also in his actions. Within moments of locking the intruder, Gerrard answered the phone and said he wouldn't be able to deliver the props in time for the play's rehearsal because he had been bothered by someone, implying the reason for Gerrard's moustaches, wigs, and other accessories in his bag.
 
Gerrard went on to say that he would use that incident in his next play, indicating that he was a dramatist, not a criminal, as he had previously said. His calm demeanour, intellect, and patience shone through in his words and actions, which, of course, rescued him from a major peril. He also requested in the call to send sergeant (Gerrard's friend) to Gerrard's cottage, who Gerrard assumed would be in the Pub. The moral of the play might also be seen as not losing one's cool in difficult situations, and remaining calm and thinking about how to get out of them can rescue us.
 
Meaning of difficult words:
 
S.No
Words
Meaning 
1.
Rattle
Sharp knocking sounds
2.
Sergeant  A police officer in UK
Reference:
National Council of Educational Research and Training (2006). Beehive. If I Were You - Doughlas James (pp. 138-144). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.