This story is very clearly influenced by The Novel of the White Powder and The Facts in the Case of M. Valdemar. I wrote about the overall commonalities of the three stories here. But I recommend reading all of them first. I'll put the spoilers there, and keep this post shorter.
Our narrator lives below an eccentric old Spanish doctor. He gradually befriends him (with shades of Erich Zann), and then things get a bit gross and weird. This is easily the best of Lovecraft's New York stories, really the only one I'd recommend to people. It is suspenseful and you really want to keep reading even if you know what is going to happen, or picked up on all the foreshadowing.
It's not typical or cliched in really any sense, despite my being able to cite two clear influences on Lovecraft here. I think the atypicalness of the story is best summarized by one of my favorite quotes from the introduction of the story:
"It is a mistake to fancy that horror is associated inextricably with darkness, silence, and solitude. I found it in the glare of mid-afternoon, in the clangor of a metropolis, and in the teeming midst of a shabby and commonplace rooming-house with a prosaic landlady and two stalwart men by my side."
Overall, a really nice, unusual piece of horror writing that shows some maturity from Lovecraft. A personal favorite, although certainly not his best.
3.5 out of 5 kinds of dark, slimy trails end in terrible little pools.