Yes, the story is very deep, but as one watches these episodes at a fast pace, with lots of faith and anticipation between every episode and wondering what would happen next, one inevitably would miss or forget some important things or clues in the story happening in Europe, at least for me. This anime, as far as I can tell, has no plotholes throughout it. It does have an open end which I won't spoil. Of course a story can always still be great with plotholes inside of it, but my point now is that everything happening in the story seemed understandable for me.
People getting influenced or manipulated, people showing of their emotions and trusts and beliefs, people's personalities feeling realistic, Monster really shows of a lot of things in people. And because of that, the viewer gets a lot of information through their head when viewing this, and almost everything of that fits somehow together, to make for a great unforgettable story for me personally.
Monster is the kind of media that makes it so rewarding on all entertainment levels. I literally found a treasure of an anime and I am absolutely glad to have seen this. It is vastly different from generic anime and I can even recommend this to Crime, Thriller and Drama fans of regular movies, given that they can stand the foreign language and the graphic style. It could very well be filmed into a regular TV series, that's how close Monster is to reality. Don't expect anything over the top to be happening in here.
And Monster is certainly not a walk in the park. There are many disturbing events playing, an example can be seen in the picture below. Victims are common in this story, as the manipulators continue to destroy everyone else around them. And as the main protagonist continues to search for the boy that changed his life, one must wonder how such a mysterious story could possibly end. But I will leave that all for the curious people to find out.
But alas, only the lucky ones will be able to get so compelled by this. And by lucky I mean no offense at all, but merely by the fact how we differ from each other. The pacing is like a regular detective story. There are many anime fans who disliked this because of its pacing, which means that they are used to lots of action happening in traditional series basically.