Now a grown woman, Adelaide Wilson (Lupita Nyong'o) and her family are revisiting her their old lake house for a vacation when exact copies of themselves, called the Tethered, mysteriously appear, seeking to replace her and her family.
This confrontation triggers Adelaide's repressed childhood memories in Santa Cruz Beach. Things immediately went haywire when Gabe (Winston Duke), Adelaide's husband, was hurt by his mirror image. The Tethered are not simply looking to replace them, they want to kill them.
Adelaide must keep her family safe from their mirror image while she finds out the truth about her past and who she is.