Content warning Spoiler!
This is a family tragedy mixed with a hint of Mark Twain's "The prince and the pauper" in a short novel that should have been probably better as a short story. One follows the slow adventures of a poor girl caught in a situation in which she crosses some wealthy people living in a mysterious building.
In the end, we understand that the whole situation was generated by rich people in need of organs for transplantation, who were recruiting poor young guys and girls with no family ties for this awful deed. Medically challenging nearing the impossible, the book revelation was hinted earlier and was pretty obvious.