Albert Camus' "The Stranger" is an exploration of alienation, absurdism, and the human condition. From its opening lines of this novella, the reader is transported into the mind of Meursault, an enigmatic protagonist whose detachment from societal norms challenges our understanding of morality and existence.
The question, from the very first page, is not "What is going to happen" - the plot of the story could be summarized in three sentences, and it is likely that many readers, including me, would have known it in advance.
Yet it is a detective story - a psychological detective story, trying to understand the protagonist, trying to cast his behaviors against the moral standards of the reader, and trying to recognize whether someone knows people with similar traits, or whether similar traits even exist in oneself.
Spoilers ahead

The first thing that becomes obvious about Meursault, the main character, is that he is alienated from his own feelings. Oh so very alienated - the opening words are:
Maman died today. Or yesterday maybe, I don't know. I got a telegram from the home: "Mother deceased. Funeral tomorrow. Faithfully yours." That doesn't mean anything. Maybe it was yesterday.
- Part I, Chapter 1
The theme of not being aware of his own feelings is immediately obvious and repeats many more times. Not so obvious, but much more damning for the character, is the lack of awareness of choices and actions, and lack of awareness of people besides himself.
Raymond, a neighbor of Meursault and a pimp, is the one setting the main conflict of the plot in motion. One evening shortly after his mother's funeral, the protagonist gets invited to dinner with him. There, Raymond complains that one of "his" girls, one that he himself is sexually involved with, is "cheating on him" - sleeping with johns and not paying him. For that, he has beaten her until she bled. Then he asks the protagonist for help:
He wanted to write her a letter, "one with a punch and also some things in it to make her sorry for what she's done." Then, when she came running back, he'd go to bed with her and "right at the last minute" he'd spit in her face and throw her out. Yes, that would punish her, I thought. But Raymond told me he didn't think he could write the kind of letter it would take and that he'd thought of asking me to write it for him. Since I didn't say anything, he asked if I'd mind doing it right then and I said no.
- Part I, Chapter 3
There's no awareness at all for the fact that then and there, he had a choice. Meursault is an observer in his own life. Another instance:
That evening Marie came by to see me and asked me if I wanted to marry her. I said it didn't make any difference to me and that we could if she wanted to. Then she wanted to know if I loved her. I answered the same way I had the last time, that it didn't mean anything but that I probably didn't love her.
- Part I, Chapter 5
Does Meursault simply not care? It is not the case:
He then asked the prosecutor if he had any questions to put to the witness, and the prosecutor exclaimed, "Oh no, that is quite sufficient!" with such glee and with such a triumphant look in my direction that for the first time in years I had this stupid urge to cry, because I could feel how much all these people hated me.
- Part II, Chapter 3
Meursault cares about how others think of him - yet he thinks that this caring is something he would like to get rid of - "this stupid urge to cry".