Theory:

INTRUDER: It’s a bag all right and this is a gun all right. What’s all this?
 
GERRARD: That’s a disguise outfit; false moustaches and what not. Now do you believe me?
 
INTRUDER: (musingly) I don’t know.
 
GERRARD: For God’s sake clear that muddled head of yours and let’s go. Come with me in the car. I can use you. If you find it’s a frame, you’ve got me in the car, and you’ve still got your gun.
 
INTRUDER: May be you’re right.
 
GERRARD: Then don’t waste time. (Goes and picks up hat and bag.)
Explanation:
 
The intruder was confused upon hearing Gerrard's words. He checked Gerrard's bag, gun and other things inside. He was clueless as to why Gerrard carried such odd stuff like artificial moustaches, wigs, and props. The intruder had no idea what was going on in Gerrard's life. Gerrard told the intruder that he was carrying a disguise costume, false moustaches, and so on in order to change his appearance to hide from the cops. He then asked the intruder whether he had trusted him.
 
Gerrard pleaded with the intruder to believe him and join him in his car. He further stated that he could be used by Gerrard in eluding the cops, and that if he found Gerrard's claims were fake, he could kill him with his gun. Fortunately for Gerrard, the intruder agreed with him and decided to accompany him. Gerrard then picked up his hat and bag to get himself ready without wasting time. He pretended to be fleeing the cops from his cottage.
 
Meaning of difficult words:
 
S.No
Words
Meaning
1.
Disguise To change the appearance
2.
MoustacheA line hair that grows on men's upper lip
3.
FrameA false story
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.