If you read the preface of this book, you'll understand the overall structure.
I stated the purpose of the research in the introductory chapter of the thesis.
This is still just the beginning of the story.
序章 refers to the introductory chapter of a book or story, whereas 序幕 specifically evokes the opening act of a stage play or a dramatic beginning.
Compound of 序 (jo, 'order' or 'beginning') and 章 (shō, 'chapter'). The kanji combination directly yields the meaning 'introductory chapter'.