Sentence:  “H'm !" he said, "so, so&mdash;it is a tragedy in a <WSD>prologue</WSD> and three acts. I am going down this afternoon to see the curtain fall for the third time on what [...] will prove a good burlesque ; but it all began dramatically enough. It was last Saturday […] that two boys, playing in the little spinney just outside Wembley Park Station, came across three large parcels done up in American cloth. […]” 
Meaning of the word marked with <WSD> tags:  A speech or section used as an introduction, especially to a play or novel.