Exploration is your passion. Discovering new and exciting locations has also excited you to no end, much more than settling down and raising a family. Nothing but the great outdoors and natural caverns can hold your interest for long, having spent days, weeks, and even years at a time searching for that one thing no one else could find. How great was it, then, that you were contacted by someone who would pay you to do just that? Not only that, but they had a lead to find a lost civilization that not even history remembered? Almost too good to be true!
Underneath a vast desert, you and a small team are sent to find any traces of this lost city. With not much to go on, most other team members are skeptical, but you're probably ecstatic beyond belief. Weeks go by, but there seems to be nothing but sand, rock, and darkness. At least the heat isn't bothering you while inside the cavern that led you this far down. That's also not to mention there's almost an endless amount of stories to be told between all the members, not a single one of which has been boring so far. All are at least hopeful of getting a big payoff if and when they do find this proof of ancient life. Little do any of them know what lies ahead of them, and what each of them will have to endure if they have any hopes of returning to the surface alive.
This plot can contain a variety of things. A race of people living underground thought to only exist in fantasy, an ancient civilization thought long extinct actually thriving outside the surface's eye, or bandits secretly operating in the shadows of the underground. This can be negotiated and discussed. A modern, steampunk, or fantasy setting can easily work for this. I plan on kidnapping, enslavement, and forced cultural acclimation to be a heavy theme here, but I am willing to hear some other interpretations to this idea, as well.