The synopsis literally tells you the whole story. Everything. There isn't a single piece of information you're missing.
What you read is what you get, although the art style did fill a bit... Ugly? More like, rushed? maybe.
It could have been a bit creepier with a better art style, and maybe without the shoujo tag on.
I feel like this review is longer than the one-shot itself but MAL won't let me post this otherwise and I always write reviews that I feel like I would have liked to check before deciding to read the manga.
Even if it's not so good it takes no time to read so I would suggest to go on anyway, but maybe don't expect the art style to go with the horror tag because it won't.