It started with a children's song and turned into a nightmare.

When Madison Hart lost her grandmother, she lost the last chance to learn about the maternal side of her family, until she came across some old photos and a bill of sale for a house in the small town of Yorke Mills. Excited by the find, Madison embarks on an adventure of discovery, only to learn a dark secret connects the town with an old mine that lies submerged beneath a lake.

A malevolent entity dwells within Lake Finnegan's waters, where the bodies of several young girls have been found over the decades. Madison is horrified to learn that the evil is set to return, and several young girls—her daughter included—are its intended victims. The entity is strong and bent on revenge, and the only person who has been able to keep it under control is too old to continue the fight.

As the days tick down to a rare Blue Moon, Madison searches for ways to break the curse and keep her daughter and future little girls of Yorke Mills alive.