It is written early on in the story that the plot is founded upon just two things. One of those things are the notes of the late Professor Angell, and the other thing is the coincidence that occurs when the narrator discovers a newspaper article.
As far as I know, he reads the diary of the rescued second mate, that he was finally able to gather from his attempted meeting. Can we all agree on this interpretation, because I was confused when I read the story last year? The Cthulhu that is depicted in the advertising is less alien like in appearance than the story version, but it is certainly cool, besides our human vision was not able to adapt to the appearance of what it was seeing.