Conrad begins the story by means of an interesting introduction which tells of how he found the tale. It reminds me a bit of how The Twilight Zone often opens:
The Find was made in a box of books bought in London, in a street which no longer exists, from a second-hand bookseller in the last stage of decay... (the story is) A litter of loose pages at the bottom of the box excited my curiosity but faintly.
I don't know if this was written by him or not. Such casting off of credit is a frequent enough occurrence that it puts authorship in doubt. Most recently, Don Quixote was written by a man who said the entire tale was stitched together from a few different sources. Perhaps for Conrad, this was an attempt at deepening the mystery behind the story. Much of the story is condensed and many details left out that he considers trivial. Perhaps it's a true story rewritten by the master. For, he writes, it's written in a fashion as dull as an inventory manifest.
The story is about a journey in Cuba. Two young men, Edgar Byrne and Tom Crobin (Cuba Tom), go on this journey, one before the other. On the journey, both come onto a hut which houses three witches: one beautiful (but evil looking) and two ugly and very old. Edgar, as he approaches this cabin, hears the voice of his friend and shipmate giving him warnings, close at hand.
He goes into the house and is told that he can stay there. He's fed some food and then lead to his room. He keeps hearing Tom warning him about something, but he cannot see him anywhere until he searches the armoire. There is Tom, stone cold dead. He examines him as best he can, and cannot find a reasonable cause of death. Just a simple bruise on his forehead and the suggestion that Tom was trying to punch someone or something.
He puts Tom in the bed meant for him, and then watches the top of the bed lower down and basically crush him. After this is done, someone comes to his door. But it is not one of the witches, it is someone who has come to help him. They discover that the reason the young witch was killing people was for the buttons on their jackets.
It's a nice little horror story, perfect before going to bed. Sweet dreams!