In Santa Cruz, California, circa 1986, a young Adelaide, portrayed by Madison Curry, embarks on a trip to the beach with her parents, Rayne and Russell (played by Anna Diop and Yahya Abdul-Mateen II). While at a carnival, Russell wins her a Michael Jackson 'Thriller' T-shirt and plays Whac-a-Mole. It's during this carefree outing that young Adelaide wanders off, encountering a homeless man holding a Jeremiah 11:11 placard. This unsettling meeting leads her to a nearby fun-house, where she is startled by her own doppelganger. The encounter leaves her traumatized, causing her to withdraw into silence, much to her parents' dismay. They seek help from a therapist who advises them to encourage Adelaide to express her feelings through creative outlets like reading or dance.
Fast forward to the present day, the now-adult Adelaide Wilson (played by Lupita Nyong'o) is on a beach retreat with her husband Gabe (portrayed by Winston Duke) and their two children, Zora (played by Shahidi Wright Joseph) and Jason (played by Evan Alex). As they settle into their beach house, it becomes apparent that this trip is meant to help the children cope with their grandmother's passing. Adelaide, still haunted by her childhood memories, shows reluctance to visit the beach. Eventually, she gives in to her family's excitement, but they witness a man being taken into an ambulance – the same homeless man from her past.
At the beach, the Wilsons connect with the Tylers, including Kitty (played by Elisabeth Moss), Josh (played by Tim Heidecker), and their twin daughters, Gwen and Maggie (played by Cali Sheldon and Noelle Sheldon). While the kids frolic in the waves, Adelaide and Kitty discuss the former's introverted nature and her history as a dancer. However, as Jason strays from the group, he encounters a man who is ominously bloodied. Panic strikes when Adelaide realizes Jason is missing, and in her frantic search, she finally locates him, deciding it's best to return home.
That night, once the family has settled, Adelaide confides in Gabe about her past trauma and expresses her fears that her doppelganger might be seeking her out. Just as Gabe reassures her, the power goes out. In the midst of their discomfort, Jason informs them of another family lurking in the driveway. When this family breaks into their home, chaos unfolds as they reveal themselves to be the Wilson family's doppelgangers, dressed in red jumpsuits and wielding scissors.
The doppelganger of Adelaide, known as Red, elaborates on a grim tale of a princess and her shadow before forcing the family into a nightmare. Zora, Jason, and even Gabe are separated and chased down by their counterparts. For instance, Zora believes she has escaped her doppelganger, Umbrae, only for her to reappear with lethal intent. Meanwhile, Abraham, Gabe's double, drags him to their boat, inciting a fight for survival that culminates in a desperate struggle at sea.
As the night wears on, the Wilson family encounters their doppelgangers in increasingly terrifying scenarios. Jason cleverly traps Pluto, the doppelganger version of himself, in a closet, while Adelaide fights for survival against Red in a harrowing instance that pits their strength and resolve against one another. The intensity of their encounters leads to deadly consequences as the Tylers fall victim to their doubles, who are equally ruthless.
Throughout the unfolding horror, news reports reveal the doppelgangers' widespread appearances and intentions. With the world unraveling, the Wilsons make a precarious escape and face numerous close calls that threaten their survival. As morning arrives, they reach the boardwalk only to confront a greater threat posed by the doppelgangers, forcing them to confront the truth about their existence.
Ultimately, Adelaide learns the shocking truth of her identity, realizing that the Red she battled was actually the original Adelaide. The film culminates in an unsettling revelation as the doppelganger rebellion is revealed to stretch across the nation, symbolizing an eerie connection between her past fears and the current nightmare unfolding around them. In the closing moments, the haunting image of all the red-clad doppelgangers holding hands echoes the themes of duality, identity, and survival.
