There are two main ways in which to play Diablo III. You can do Adventure Mode, and spend your time completing bounties and running through Rifts. Or, you can do Story Mode, go through the game Act by Act, and experience the story chronologically.
I suspect that Adventure Mode is the more popular of the two options. It’s faster. It gives players a more efficient way to level a character. It could very well be the best way to find the “loot” you have been seeking.
Story Mode gives the player the overall story that involves the main characters, as well as some history. In addition, there’s a bunch of smaller stories in it just waiting to be found. One of those stories is about an NPC (non-player character) named Morgan that the player never actually gets to meet. All we have to go on are some pages from his journal.