We get a long, past-tense description of the strangeness of the titular house, and how everyone who lived there seems to die an early death. We learn that some residents spoke in French before dying without ever having know French. And we learn all about the family history and renovations made to the house. Lovecraft lets slip that not only does the narrator survive, but the mysterious problem has been solved and now people don't die or have anything weird happen in the house. I'm inclined to either stop here, or fall asleep.
The narrator tosses around a few theories about vampires and werewolves, and tells of previous residents trying to kill their own families and being found drained of blood. Eventually he sees a human-like gaseous form rise from the fungus in the cellar, and he and his uncle decide to go in, watch the spot, and attempt to eliminate the threat. This whole story has a distinct feel of the Call of Cthulhu pencil and paper RPG. And the main characters seem a bit munchkiny.
They bring two absurd methods of fighting the unknown thing (which they take some wild guesses at that Lovecraft implies to have been not too far off): a pair of WWI flamethrowers, and a giant Crookes Tube (LASER). They have an option for either the corporeal or incorporeal menace with little reason to how they got such ridiculous bullshit.
Anyway, they have an interesting and tense night in the house and eventually vanquish the monster, which ends up a little weirder and more difficult than anyone, including me, had expected. The night vigil is actually scary and interesting and I like the sciencey method of victory. I do quite like the last two chapters and especially the night spent in the house, but overall the story leaves a lot to be desired. 2.5 out of 5.
P.S. the most significant thing in this story is the beginning of Lovecraft's use of scientific justifications and solutions to his supernatural problems. We'll see a lot more of this later on.