Sanctuary. A very long term shelter built inside a natural network of caves. Its original intention was to be a safe place for people to survive if there was a nuclear war, but it wasn't nukes that forced people to hastily move into Sanctuary for good generations ago. Instead it was an invasion from somewhere far from Earth, an invasion that humanity was completely unprepared for.

Now, Sanctuary is the only home the descendants of the people who made it there that day have ever known. They may well be the last of humanity, and Curtis, well, he is going to be the last of them. After his mother died in childbirth over two decades ago, the inhabitants of Sanctuary realized that they weren't equipped to deal with reproduction anymore, and promptly made any act that could lead to pregnancy forbidden, with the strictest of punishments.

Naturally, this made Sanctuary a pretty dismal and frustrating place to be a young, red-blooded man in, though it isn't until Curtis is under the threat of one of these 'strict punishments' that events conspire to send him out into the world outside of the caves.

With no idea whether the surface is an empty wilderness, whether the alien invaders now rule over the lands, or whether humanity somehow survived and rebuilt, Curtis must discover what became of the old world, and find a way to survive - maybe even to claim a future for himself. But what will he find when he leaves behind his oppressive existence, and ventures into the unknown?

A harem. Come on, it's not a spoiler - it's in the series title. He gets a harem. With androids in it.