It has been ten months since forest conservationist Sienna lost her daughter in a tragic home accident when she was the only other person around. Kira's death transforms Sienna into an overprotective mother to her son. But then Kai dies under similar circumstances. Desperate to keep from losing her mind, if only to preserve the memories of her children, Sienna climbs into the treehouse where Kai took his last breath.

While meditating to connect with him, terrifying images of a "mother tree," a nurturing hub tree Sienna studied in an isolated meadow near their lake house, corrupt all memories of her kids. No longer a majestic, colorful sentinel, the tree menaces her with its limbs draped in leaves that morph into skeletal hands. As Sienna's thoughts fester, convincing her that her children's deaths were not simple accidents, those around her try to persuade her she must be guilty of harming Kira and Kai, even if she can't remember. Anxious to unveil the terrible truth hiding in her psyche, Sienna instead finds herself in very physical danger as secrets come to light.