Is it normal to start reading a book but then suddenly realize that you have no idea what's going on in the plot because you have been reading the words but ignoring their meaning?

ADHD aside, you need to ask yourself if you like it, if you're really interested in the content shown, if maybe your mind is distracted in something, or you're worried/uncomfortable in the environment you are in and the likes. Have you read How to Read a Book by Mortimer J. Adler? the author explains that we learn to read just on a surface level, leading sometimes to problems like you express. He then teaches how to approach a book in a way that you squeeze the most information and knowledge from it, even if its just a fiction book; and in the process, remember everything that you read even after finishing it. What works for me is: give a conscious effort to concentrate in what you're reading, but don't stay in the same page, don't even read a sentence two times. See if you concentrate better with music; I have ADHD and when I read books I need to listen to three diferent playlists to concentrate. Finally, following the first thing I mentioned, giving a conscious effort is exhausting to the brain so when you realize you can't concentrate anymore just leave it and start again the next day where you left it, you will notice that your brain will stand more and more all the work out you're giving it.