Four friends find a mystery-a hatchway set in the ground at their feet. Where did in come from, and where could it go? Fenced off from the world, the only four children living at a remote science facility had gotten used to relying on each other. Their parents, preoccupied with their research, had left the four of them to grow up on their own, finding their own adventures and figuring things out for themselves. Sure, things had been getting a little strange lately. But a few people going missing wasn't much to worry about when there's barely any school and plenty of sunny days to explore. But this hatch, or door, or whatever it was-this was new, and unexpected, and unexplainable. Honey didn't trust it. Ip was eager to jump down, but even his big brother Ralsh couldn't pull it open. Janus knew he'd be able to figure it out, somehow. As the friends try to solve the puzzle, more startling events make them wonder about the doorway, and what might be going on under their feet. Their parents were acting more and more... oddly. And why would a research station need so many soldiers? Their questions become more serious when the last mystery falls into place: Every single person in their lives, all but the four of them, vanishes. Opening the hatch and dropping into the darkness below may be the only way to find their families. And whatever else is down there.