Sixteen-year-old Libby Wells returns from summer camp to the surprise introduction of her Mom's fiancée, John Smith, who emerges from the backyard lake, chiseled and glistening in the afternoon light. While her Mom nurses a foreign illness in the upstairs bedroom, John ignites a sexual curiosity in Libby and she slowly accepts his awkward, confusing advances. One night, Libby wakes to a piercing, amber light that cascades into her room from the depths of the lake outside. She spots John on the shore, basking in the luminance, until he walks into the water, completely submerges and then.... disappears as the light clicks off. Immediately, Libby questions John's outward perfection. Her suspicions raised, she quickly uncovers his true nature and sinister strategy, even as her Mother clings to his infectious charm.