After a family tragedy, Emily, Navin, and their mother move to an old ancestral home to start a new life. On the family's very first night in the mysterious house, a strange noise lures them into the basement, where Em and Navin's mom is kidnapped by a humongous, tentacled creature and dragged down behind the basement door.

The kids give chase down a twisty spiral stairway and find themselves in a strange and magical world below. Most surprising of all, it seems that their great-grandfather, who was an inventor and puzzle maker, was there before them – and he's left some unfinished business.

Now it's up to Em and Navin to figure out how to set things right and save their mother's life!