Carol White, portrayed by Julianne Moore, lives with her husband Greg, played by Xander Berkeley, and their son in the picturesque San Fernando Valley. On the surface, Carol appears to lead an idyllic life; however, she soon begins to suffer from a series of unexplained ailments that leave her feeling increasingly ill at ease with her surroundings. Her initial attempts to rectify her condition include changing her diet and lifestyle, but no matter what she tries, her physician cannot identify any medical issues.

When her condition takes a turn for the worse, resulting in a collapse while driving home, Carol consults a doctor who uncovers a mild allergy. As her situation grows more desperate, Carol seeks to uncover the real cause behind her mysterious illness, but both doctors and psychologists are baffled—further tests suggest she has no actual allergies. Struggling with this uncertainty, she adopts an extreme diet recommended by her friend Nell, but ultimately begins to point fingers at a myriad of environmental factors including chemicals found in food, cleaning supplies, and pollutants.

The pivotal moment occurs during a party held in honor of a close friend who is heavily pregnant, where Carol experiences a significant emotional outburst. Driven by her need for answers, she discovers a retreat named Wrenwood, a sanctuary where others suffering from various illnesses come to heal. This retreat is governed by a charismatic doctor encouraging the patients to introspect as a pathway to recovery. Carol quickly learns that the rules there are stringent: all clothing must be made of cotton, only organic food and drink are allowed, and cars are strictly banned—much to her chagrin upon her first arrival.

In Wrenwood, she encounters a couple who have also retreated for health reasons. They have created a secluded bunker, or igloo, meant to regulate their environment, but tragedy looms as one of them, despite their efforts, faces impending death. As Carol adjusts to her new surroundings, she discovers a highway not far from the compound, prompting her request for relocation within Wrenwood. The interactions with Greg and their child grow increasingly strained and cold, very much devoid of any physical affection, illustrating the burden her illness has placed on her family.

Eventually, Carol makes the decision to cut ties with her family completely, seeking solace in the solitude of her isolated igloo. Here, she can finally face her inner turmoil and unresolved feelings. The film culminates with a poignant scene of Carol gazing into a mirror and softly uttering the words, > “I love you,” as her reflection reveals a profound sadness and a sense of desolation.