Basically the story talks about how Sadie serve her “master” to cultivate fresh flesh to “wear”, while eventually Sadie cannot stand the burden of seeing all those children being taken. She disobey the order and sacrifice herself instead of its expected container, merged the “master” into her body.
Personally I do not think there is something like a soul of human, where most people would believe to be a spirit that was stored in our mind. The extraction of spirit and inject it into another body has been considered to be a way to remain immortal or at least a way to face aging and accidental death.
Nonetheless, I think it is highly possible that human memory could be restored like the computer as a digitalized data that could be transmit or put into another body one day. Our cells and nerves contains bio-electricity that reflects informations that we may not be able to decode nowadays just like we yet have insufficient technologies to analysis electroencephalography, electromyography or electrocardiography.
To cultivate an organism as a container for a process of “putting old coal not new body” would one day be possible if human beings finally can translate our memory into visible signals. Though there are moral issues, or cause another issue of population control if we still have new-borns, there is always a solution - like building robots to be the containers, which could be designed to simulate every single function of human beings.