So let's enumerate some things:
-pinkish, crab-like aliens with bat-like wings fly through the ether and reach Earth
-they supposedly mine rocks from Earth
-they are "the old ones", so they're far older than the race of man
-Lovecraft insists via his characters (including his main character who starts out as a rational man, but does quickly a 180 and becomes a crackpot) that these crab-like ancient aliens have human-like agents working for them, that they possess psychic powers
-they come from the unknown parts of the world/universe, parts which AUTOMATICALLY in Lovecraft's mind, must be places of "cosmic horror" and any attempt to understand such places or see such places is AUTOMATICALLY blasphemous & will bring insanity to the human mind
Now comes the fun part:
-dogs, apparently, are able to keep these creatures at bay
-though these aliens are far older than man & more powerful, they apparently haven't learned by now how to properly use their wings to fly on Earth - but they're learning to improve their flight as the dull & static exchange of letters between the two men drags on for ages
-a flood can kill these eldritch "horrors"
-Lovecraft repeats around 5 to 6 times why the bloke doesn't want to leave the cabin, because that's the place of his cherished childhood and where 6 generations lived; Lovecraft seems to press so much this (bullshit) motivation in order to defend/explain why the character chooses to remain alone, with his dogs, suffering bullet attacks against his home & aliens "closing in" on him - instead of fleeing for a safe life to live with his son
-There's nothing remotely cosmic about all of this. Indeed, there's nothing horrific about it as well. The bulk of the story is one non-sequitur after another. It seems like a bad episode of (pseudo-documentary) Ancient Aliens - granted, ahead of that series by many decades & fatalistic without any sort of good argument, whilst the people on the Ancient Aliens show are very lively and optimistic. I'm only familiar with Call of Cthulhu, The Horror at Red Hook, The Whisperer in Darkness, and The Thing on the Doorstep - and only the latter I enjoyed (the lovely voice actor Wayne June helped a lot). For a further LOL, unrelated to this particular story, Lovecraft's inspiration for The Horror at Red Hook was this:
Lovecraft referred to the area's immigrant population by referring to Red Hook as "a maze of hybrid squalor". He spelled out his inspiration for "The Horror at Red Hook" in a letter written to fellow writer Clark Ashton Smith:https://en.wikipedia.org/wiki/The_Horror_at_Red_Hook
The idea that black magic exists in secret today, or that hellish antique rites still exist in obscurity, is one that I have used and shall use again. When you see my new tale "The Horror at Red Hook", you will see what use I make of the idea in connexion with the gangs of young loafers & herds of evil-looking foreigners that one sees everywhere in New York.
We can only imagine what inspiration Lovecraft would have if he were alive today, with this whole Syrian refugee crisis.