John James (Kevin Costner) moves with his two children Louisa (Ivana Baquero) and Sam (Gattlin Griffith) to a new house in South Carolina because his wife has left them.
The place is quite isolated and Louisa begins to get fascinated with a mound next to the house. Soon she begins to show a strange behavior. At the same time, strange things start happening.
Louisa visits the mound often, staying late and coming home covered with mud.
John is worried about his daughter. Trying to find a match between the strange behavior and the mound he searchs the internet finding an expert in ancient civilizations and ask him for help.
Professor Evan White (Noah Taylor) reveals the mound is really an Indian burial mound. It was some kind of place where deities of an ancient civilization called mound walkers used to live. These deities through a strange ritual used to pick a human to avoid extinction. Because the walkers were an all-male race, they would need a young human female with whom to mate. So we discover that Louisa is the chosen one who would gave birth to the next generation of walkers.
In the end the walkers enter the house and take Louisa inside the mound so she would give birth when her time came.
John asks his son to stay in the house until the police get there.
After emptying a can of gasoline round the mound he crawls into one of the openings there to find a labyrinth of tunnels. He finds Louisa and make their way outside the tunnel while they are pursued by the walkers.
John blocks the tunnel with a gasoline barrel and lights a flare. Another walker appears in this moment to protect his queen and John can see how her daughter has begun to transform into one of the creatures. John drops the flare and everything explodes.
Sam is outside the house watching it all and carrying a photo in his arms. After the explosion we can see in the glass's reflection a figure walking out of the fire but we can also see three mound walkers approaching Sam from behind. In the final scene Sam asks:"Daddy?" But we only hear a screech as reply. It's not shown who is walking out of the fire or what finally happens to Sam.